Update buildroot and opt out of LTO on unopt builds. (flutter/engine#3952)

DEPS

@@ -75,7 +75,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '6b9d76de34c1529063c927d663e44104448fb778',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '4cf93efdd098f6c3bc26b9f38f2306fa0560ead6',
 
    # Fuchsia compatibility
    #

engine/src/flutter/shell/platform/darwin/ios/BUILD.gn

@@ -5,7 +5,6 @@
 assert(is_ios)
 
 import("//flutter/common/config.gni")
-import("//build/config/ios/rules.gni")
 import("//build/config/ios/ios_sdk.gni")
 
 _flutter_framework_dir = "$root_out_dir/Flutter.framework"

engine/src/flutter/tools/gn

@@ -68,6 +68,7 @@ def to_gn_args(args):
     gn_args['skia_use_fontconfig'] = False # Use the custom font manager instead.
 
     gn_args['is_debug'] = args.unoptimized
+    gn_args['enable_lto'] = not args.unoptimized
     gn_args['android_full_debug'] = args.target_os == 'android' and args.unoptimized
     gn_args['is_clang'] = not sys.platform.startswith(('cygwin', 'win'))