cc: Change required_for_activation bookkeeping.

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index d6d20a6509e4422b58187bafe79c3b60ac27ad1f..1f07667fea725a9a2f58e07cefb17700f1cafa48 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -51,6 +51,7 @@ class QuadSink;
 class Renderer;
 class ScrollbarAnimationController;
 class ScrollbarLayerImplBase;
+class Tile;
 
 struct AppendQuadsData;
 
@@ -178,6 +179,7 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
   virtual RenderPass::Id NextContributingRenderPassId(RenderPass::Id id) const;
 
   virtual void UpdateTilePriorities() {}
+  virtual void NotifyTileInitialized(const Tile* tile) {}
 
   virtual ScrollbarLayerImplBase* ToScrollbarLayer();