move cleanup code to EndFrame

Author: Chris Yang

shell/common/rasterizer.cc

@@ -190,8 +190,7 @@ void Rasterizer::Draw(fml::RefPtr<Pipeline<flutter::LayerTree>> pipeline,
     consume_result = PipelineConsumeResult::MoreAvailable;
   }
 
-  // Merging the thread as we know the next `Draw` should be run on the platform
-  // thread.
+  // EndFrame should perform cleanups for the external_view_embedder.
   if (surface_ != nullptr && surface_->GetExternalViewEmbedder() != nullptr) {
     surface_->GetExternalViewEmbedder()->EndFrame(should_resubmit_frame,
                                                   raster_thread_merger_);

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -554,8 +554,6 @@
   BringLayersIntoView(platform_view_layers);
   // Mark all layers as available, so they can be used in the next frame.
   layer_pool_->RecycleLayers();
-  // Reset the composition order, so next frame starts empty.
-  composition_order_.clear();
 
   did_submit &= frame->Submit();
 
@@ -595,7 +593,10 @@
 
 void FlutterPlatformViewsController::EndFrame(
     bool should_resubmit_frame,
-    fml::RefPtr<fml::RasterThreadMerger> raster_thread_merger) {}
+    fml::RefPtr<fml::RasterThreadMerger> raster_thread_merger) {
+  // Reset the composition order, so next frame starts empty.
+  composition_order_.clear();
+}
 
 std::shared_ptr<FlutterPlatformViewLayer> FlutterPlatformViewsController::GetLayer(
     GrDirectContext* gr_context,