Make GetDefaultFontFamilies return a vector<string> instead of… (flutter/engine#16928)

On Linux, there is rarely just one default font that can reasonably be expected to be on the platform. This PR changes the GetDefaultFontFamily call to be GetDefaultFontFamilies, and it now returns a vector<string> so that the font collection code can look up all of them, and if any of them exist, add them to the fallback list.

For Linux, I supplied the list "Ubuntu", "Cantarell", "DejaVu Sans", "Liberation Sans", and "Arial", which should cover a large proportion of linux machines. For the other platforms, I supplied a list of length one, containing the one fallback font that used to be defined. On Windows, I added "Segoe UI" as a default, since that is the default system font on newer Windows.

The goal of this function is to provide at least one font family that is installed, since otherwise linux (or any platform) will just have no font at all if the default font isn't found.

engine/src/flutter/shell/platform/linux/BUILD.gn

@@ -16,7 +16,7 @@ group("linux") {
   }
 }
 
-# Temporary workraround for the issue describe in
+# Temporary workaround for the issue describe in
 # https://github.com/flutter/flutter/issues/14509 and
 # https://github.com/flutter/flutter/issues/14438
 # Remove once the build infrastructure moves to Ubuntu 18.04 or newer, where

engine/src/flutter/third_party/txt/src/txt/font_collection.cc

@@ -150,11 +150,14 @@ FontCollection::GetMinikinFontCollectionForFamilies(
   }
   // Search for default font family if no user font families were found.
   if (minikin_families.empty()) {
-    const auto default_font_family = GetDefaultFontFamily();
-    std::shared_ptr<minikin::FontFamily> minikin_family =
-        FindFontFamilyInManagers(default_font_family);
-    if (minikin_family != nullptr) {
-      minikin_families.push_back(minikin_family);
+    const auto default_font_families = GetDefaultFontFamilies();
+    for (auto family : default_font_families) {
+      std::shared_ptr<minikin::FontFamily> minikin_family =
+          FindFontFamilyInManagers(family);
+      if (minikin_family != nullptr) {
+        minikin_families.push_back(minikin_family);
+        break;
+      }
     }
   }
   // Default font family also not found. We fail to get a FontCollection.
@@ -319,7 +322,7 @@ FontCollection::CreateSktFontCollection() {
     skt_collection_ = sk_make_sp<skia::textlayout::FontCollection>();
 
     skt_collection_->setDefaultFontManager(default_font_manager_,
-                                           GetDefaultFontFamily().c_str());
+                                           GetDefaultFontFamilies()[0].c_str());
     skt_collection_->setAssetFontManager(asset_font_manager_);
     skt_collection_->setDynamicFontManager(dynamic_font_manager_);
     skt_collection_->setTestFontManager(test_font_manager_);

engine/src/flutter/third_party/txt/src/txt/platform.cc

@@ -6,8 +6,8 @@
 
 namespace txt {
 
-std::string GetDefaultFontFamily() {
-  return "Arial";
+std::vector<std::string> GetDefaultFontFamilies() {
+  return {"Arial"};
 }
 
 sk_sp<SkFontMgr> GetDefaultFontManager() {

engine/src/flutter/third_party/txt/src/txt/platform.h

@@ -6,13 +6,15 @@
 #define TXT_PLATFORM_H_
 
 #include <string>
+#include <vector>
+
 #include "flutter/fml/macros.h"
 
 #include "third_party/skia/include/core/SkFontMgr.h"
 
 namespace txt {
 
-std::string GetDefaultFontFamily();
+std::vector<std::string> GetDefaultFontFamilies();
 
 sk_sp<SkFontMgr> GetDefaultFontManager();
 

engine/src/flutter/third_party/txt/src/txt/platform_android.cc

@@ -6,8 +6,8 @@
 
 namespace txt {
 
-std::string GetDefaultFontFamily() {
-  return "sans-serif";
+std::vector<std::string> GetDefaultFontFamilies() {
+  return {"sans-serif"};
 }
 
 sk_sp<SkFontMgr> GetDefaultFontManager() {

engine/src/flutter/third_party/txt/src/txt/platform_fuchsia.cc

@@ -6,8 +6,8 @@
 
 namespace txt {
 
-std::string GetDefaultFontFamily() {
-  return "Roboto";
+std::vector<std::string> GetDefaultFontFamilies() {
+  return {"Roboto"};
 }
 
 sk_sp<SkFontMgr> GetDefaultFontManager() {

engine/src/flutter/third_party/txt/src/txt/platform_linux.cc

@@ -12,8 +12,8 @@
 
 namespace txt {
 
-std::string GetDefaultFontFamily() {
-  return "Arial";
+std::vector<std::string> GetDefaultFontFamilies() {
+  return {"Ubuntu", "Cantarell", "DejaVu Sans", "Liberation Sans", "Arial"};
 }
 
 sk_sp<SkFontMgr> GetDefaultFontManager() {

engine/src/flutter/third_party/txt/src/txt/platform_mac.mm

@@ -16,11 +16,11 @@
 
 namespace txt {
 
-std::string GetDefaultFontFamily() {
+std::vector<std::string> GetDefaultFontFamilies() {
   if (fml::IsPlatformVersionAtLeast(9)) {
-    return [FONT_CLASS systemFontOfSize:14].familyName.UTF8String;
+    return {[FONT_CLASS systemFontOfSize:14].familyName.UTF8String};
   } else {
-    return "Helvetica";
+    return {"Helvetica"};
   }
 }
 

engine/src/flutter/third_party/txt/src/txt/platform_windows.cc

@@ -7,8 +7,8 @@
 
 namespace txt {
 
-std::string GetDefaultFontFamily() {
-  return "Arial";
+std::vector<std::string> GetDefaultFontFamilies() {
+  return {"Segoe UI", "Arial"};
 }
 
 sk_sp<SkFontMgr> GetDefaultFontManager() {

engine/src/flutter/third_party/txt/src/txt/text_style.cc

@@ -23,8 +23,7 @@
 
 namespace txt {
 
-TextStyle::TextStyle()
-    : font_families(std::vector<std::string>(1, GetDefaultFontFamily())) {}
+TextStyle::TextStyle() : font_families(GetDefaultFontFamilies()) {}
 
 bool TextStyle::equals(const TextStyle& other) const {
   if (color != other.color)