Initial attempt at counting layers in the compositor thread.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 09449dd64ccede6c578b5f6d0c33b6b640ec4742..b2520339a4a1468b71970eb0ec37172b860bd93a 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -464,6 +464,9 @@ class CC_EXPORT LayerTreeHostImpl
   void RegisterPictureLayerImpl(PictureLayerImpl* layer);
   void UnregisterPictureLayerImpl(PictureLayerImpl* layer);
 
+  // Public only for for unit testing

[danakj, 2014/07/10 18:03:41]

comments are full sentences with periods

+  size_t num_layers_for_testing() { return num_layers_; }
+
  protected:
   LayerTreeHostImpl(
       const LayerTreeSettings& settings,
@@ -696,6 +699,11 @@ class CC_EXPORT LayerTreeHostImpl
 
   std::vector<PictureLayerImpl*> picture_layers_;
 
+  // The number of layers on the most recent frame,
+  // whether or not they were drawn.
+  // Default value is 0.
+  size_t num_layers_;

[dneto, 2014/07/10 01:29:03]

Not sure if size_t is appropriate here, or overkill.  I went with size_t because
the Chromium style guide says to use size_t when counting things. To be honest
unsigned would feel more natural.

[Ian Vollick, 2014/07/10 03:16:23]

I agree that this is overkill, but there's no harm in using a size_t for style's
sake. I think this is fine.

[danakj, 2014/07/10 18:03:41]

"unsigned" is generally banned. You'd use an int otherwise (we use unsigned for
GL data types actually, or uint32 but that's one of few exceptions).

[dneto, 2014/07/10 18:44:16]

Acknowledged.

[dneto, 2014/07/10 18:44:17]

Acknowledged.

+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };