ui: Cleanup class/struct forward declarations

services/ui/public/cpp/window_compositor_frame_sink.h

Index: services/ui/public/cpp/window_compositor_frame_sink.h
diff --git a/services/ui/public/cpp/window_compositor_frame_sink.h b/services/ui/public/cpp/window_compositor_frame_sink.h
index 2bb98687125879acab6f7bc721457e96b6af9651..e8fd7c5b89bfc2e9575c083d4583b20f265c6b88 100644
--- a/services/ui/public/cpp/window_compositor_frame_sink.h
+++ b/services/ui/public/cpp/window_compositor_frame_sink.h
@@ -23,7 +23,8 @@ class WindowCompositorFrameSinkBinding;
 
 class WindowCompositorFrameSink
     : public cc::CompositorFrameSink,
-      public cc::mojom::MojoCompositorFrameSinkClient {
+      public cc::mojom::MojoCompositorFrameSinkClient,
+      public cc::ExternalBeginFrameSourceClient {
  public:
   // static
   static std::unique_ptr<WindowCompositorFrameSink> Create(
@@ -51,9 +52,13 @@ class WindowCompositorFrameSink
 
   // cc::mojom::MojoCompositorFrameSinkClient implementation:
   void DidReceiveCompositorFrameAck() override;
+  void OnBeginFrame(const cc::BeginFrameArgs& begin_frame_args) override;
   void ReclaimResources(const cc::ReturnedResourceArray& resources) override;
 
-  std::unique_ptr<cc::BeginFrameSource> begin_frame_source_;
+  // cc::ExternalBeginFrameSourceClient implementation.
+  void OnNeedsBeginFrames(bool needs_begin_frames) override;
+
+  std::unique_ptr<cc::ExternalBeginFrameSource> begin_frame_source_;
   mojo::InterfacePtrInfo<cc::mojom::MojoCompositorFrameSink>
       compositor_frame_sink_info_;
   mojo::InterfaceRequest<cc::mojom::MojoCompositorFrameSinkClient>