cc: Allow the main thread to cancel commits

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index af52b6dfe83bd0e6a140daaca5633986c9975d88..979472cfa6b0d9406c44ab477e7253fe9741365e 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -32,6 +32,7 @@ class SingleThreadProxy : public Proxy, LayerTreeHostImplClient {
   virtual void CreateAndInitializeOutputSurface() OVERRIDE;
   virtual const RendererCapabilities& GetRendererCapabilities() const OVERRIDE;
   virtual void SetNeedsAnimate() OVERRIDE;
+  virtual void SetNeedsUpdateLayers() OVERRIDE;
   virtual void SetNeedsCommit() OVERRIDE;
   virtual void SetNeedsRedraw(gfx::Rect damage_rect) OVERRIDE;
   virtual void SetDeferCommits(bool defer_commits) OVERRIDE;
@@ -113,6 +114,8 @@ class SingleThreadProxy : public Proxy, LayerTreeHostImplClient {
 
   bool inside_draw_;
 
+  bool can_cancel_commit_;
+
   DISALLOW_COPY_AND_ASSIGN(SingleThreadProxy);
 };