Make the Mac target deps look similar to iOS

Fixes Mac builds

engine/src/flutter/BUILD.gn

@@ -8,8 +8,8 @@ group("default") {
   deps = [
     "//sky",
   ]
-  if (!is_ios) {
-    # Mojo shell does not exist on ios
+  if (!is_ios && !is_mac) {
+    # Mojo shell does not exist on iOS or Mac
     deps += [ "//services/sky" ]
   }
 }

engine/src/flutter/sky/dist/BUILD.gn

@@ -24,7 +24,7 @@ copy("sky_shell") {
     deps = [
       "//sky/shell",
     ]
-  } else if (is_ios) {
+  } else if (is_ios || is_mac) {
     sources = [
       "$root_build_dir/Sky.app",
     ]
@@ -99,7 +99,7 @@ group("dist") {
     ":sky_viewer",
   ]
 
-  if (!is_ios) {
+  if (!is_ios && !is_mac) {
     deps += [ ":sky_shell" ]
   }