Don't compile the Skia GL backend on iOS. (flutter#34924)

shell/common/shell_io_manager.cc

@@ -13,20 +13,20 @@ namespace flutter {
 sk_sp<GrDirectContext> ShellIOManager::CreateCompatibleResourceLoadingContext(
     GrBackend backend,
     sk_sp<const GrGLInterface> gl_interface) {
+#if !OS_FUCHSIA && SK_GL
   if (backend != GrBackend::kOpenGL_GrBackend) {
     return nullptr;
   }
 
   const auto options = MakeDefaultContextOptions(ContextType::kResource);
 
-#if !OS_FUCHSIA
   if (auto context = GrDirectContext::MakeGL(gl_interface, options)) {
     // Do not cache textures created by the image decoder.  These textures
     // should be deleted when they are no longer referenced by an SkImage.
     context->setResourceCacheLimit(0);
     return context;
   }
-#endif
+#endif  // !OS_FUCHSIA && SK_GL
 
   return nullptr;
 }

tools/gn

@@ -398,12 +398,16 @@ def to_gn_args(args):
 
   # Enable Metal on iOS builds.
   if args.target_os == 'ios':
-    gn_args['skia_use_metal'] = True
+    gn_args['shell_enable_gl'] = False
+    gn_args['skia_use_gl'] = False
     gn_args['shell_enable_metal'] = True
+    gn_args['skia_use_metal'] = True
     # Bitcode enabled builds using the current version of the toolchain leak
     # C++ symbols decorated with the availability attribute. Disable these
     # attributes in release modes till the toolchain is updated.
     gn_args['skia_enable_api_available_macro'] = args.runtime_mode != 'release'
+  else:
+    gn_args['skia_use_gl'] = args.target_os != 'fuchsia'
 
   if sys.platform == 'darwin' and args.target_os not in ['android', 'fuchsia',
                                                          'wasm']: