[embedder] FlutterBackingStoreConfig allows special treatment of root-views

shell/platform/embedder/embedder.h

@@ -871,6 +871,8 @@ typedef struct {
   size_t struct_size;
   /// The size of the render target the engine expects to render into.
   FlutterSize size;
+  /// This is true if a backing store is requested for the root view.
+  bool is_root_view;
 } FlutterBackingStoreConfig;
 
 typedef enum {

shell/platform/embedder/embedder_external_view_embedder.cc

@@ -119,13 +119,15 @@ SkCanvas* EmbedderExternalViewEmbedder::CompositeEmbeddedView(int view_id) {
 }
 
 static FlutterBackingStoreConfig MakeBackingStoreConfig(
-    const SkISize& backing_store_size) {
+    const SkISize& backing_store_size,
+    bool is_root_view) {
   FlutterBackingStoreConfig config = {};
 
   config.struct_size = sizeof(config);
 
   config.size.width = backing_store_size.width();
   config.size.height = backing_store_size.height();
+  config.is_root_view = is_root_view;
 
   return config;
 }
@@ -172,8 +174,8 @@ void EmbedderExternalViewEmbedder::SubmitFrame(
     // directly.
     const auto render_surface_size = external_view->GetRenderSurfaceSize();
 
-    const auto backing_store_config =
-        MakeBackingStoreConfig(render_surface_size);
+    const auto backing_store_config = MakeBackingStoreConfig(
+        render_surface_size, external_view->IsRootView());
 
     // This is where the embedder will create render targets for us. Control
     // flow to the embedder makes the engine susceptible to having the embedder

shell/platform/embedder/tests/embedder_config_builder.cc

@@ -276,7 +276,6 @@ void EmbedderConfigBuilder::SetCompositor() {
     return reinterpret_cast<EmbedderTestCompositor*>(user_data)->Present(
         layers,       //
         layers_count  //
-
     );
   };
   project_args_.compositor = &compositor_;