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 04bd4a336bec02a068c91da89ef8eef57281e283..0c32f9f041db0d1e398eef8f80a246c89e274f05 100644
--- a/cc/output/output_surface.h
+++ b/cc/output/output_surface.h
@@ -135,6 +135,15 @@ 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.
+  //
+  // OutputSurfaceClient::RequestDraw is called by the output surface later to
+  // notify that the output surface is ready for a draw.
+  virtual void Invalidate() {}
+
  protected:
   OutputSurfaceClient* client_;