Keeping track of descendants that draw content instead of recalcualting

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 979d2e02c8c0a96ce4ce13b2de5a5c38f20ba5c3..b9713e1e8d7c160cc6cb4672a720c4ca6270e552 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -139,6 +139,7 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
     return scroll_children_.get();
   }
 
+  void SetNumDescendantsThatDrawContent(int num_descendants);
   void SetClipParent(LayerImpl* ancestor);
 
   LayerImpl* clip_parent() {
@@ -210,6 +211,7 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
   void SetDrawsContent(bool draws_content);
   bool DrawsContent() const { return draws_content_; }
 
+  virtual int NumDescendantsThatDrawContent() const;
   void SetHideLayerAndSubtree(bool hide);
   bool hide_layer_and_subtree() const { return hide_layer_and_subtree_; }
 
@@ -650,6 +652,8 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
   gfx::Vector2d sent_scroll_delta_;
   gfx::Vector2dF last_scroll_offset_;
 
+  int num_descendants_that_draw_content_;
+
   // The global depth value of the center of the layer. This value is used
   // to sort layers from back to front.
   float draw_depth_;