cc: Make scheduling be driven by vsync for android webview.

cc/output/output_surface.h

Index: cc/output/output_surface.h
diff --git a/cc/output/output_surface.h b/cc/output/output_surface.h
index 29e32a5317ef3c61f2b329f6a390d1eb0d0f115f..0c1ff4a4bb7aa094b299f812fb089187834f127c 100644
--- a/cc/output/output_surface.h
+++ b/cc/output/output_surface.h
@@ -143,6 +143,12 @@ class CC_EXPORT OutputSurface {
   void DidLoseOutputSurface();
   void SetMemoryPolicy(const ManagedMemoryPolicy& policy);
 
+  // Support for a pull-model where draws are requested by the output surface.
+  //
+  // OutputSurface::Invalidate is called by the compositor to notify that
+  // there's new content.
+  virtual void Invalidate() {}
+
  protected:
   OutputSurfaceClient* client_;