cc: Introduce BeginFrame sequence numbers and acknowledgements.

content/browser/compositor/mus_browser_compositor_output_surface.h

Index: content/browser/compositor/mus_browser_compositor_output_surface.h
diff --git a/content/browser/compositor/mus_browser_compositor_output_surface.h b/content/browser/compositor/mus_browser_compositor_output_surface.h
index 5f9048ca483b167c17271cb1e4978d5f78d09c9d..053d1b79f4cf5072ea967ab10f0cfd62117aea46 100644
--- a/content/browser/compositor/mus_browser_compositor_output_surface.h
+++ b/content/browser/compositor/mus_browser_compositor_output_surface.h
@@ -58,6 +58,8 @@ class MusBrowserCompositorOutputSurface
 
   ~MusBrowserCompositorOutputSurface() override;
 
+  cc::BeginFrameSource* GetBeginFrameSource();
+
  protected:
   // cc::OutputSurface implementation.
   void SwapBuffers(cc::OutputSurfaceFrame frame) override;
@@ -89,6 +91,7 @@ class MusBrowserCompositorOutputSurface
   std::unique_ptr<aura::WindowCompositorFrameSink> compositor_frame_sink_;
   std::vector<uint32_t> free_resource_ids_;
   std::vector<gpu::Mailbox> mailboxes_;
+  cc::BeginFrameSource* begin_frame_source_;
 
   DISALLOW_COPY_AND_ASSIGN(MusBrowserCompositorOutputSurface);
 };