UMA metrics for use count of wheel and touch scrolls.

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 6e52d0ffb7dd2f92dc25baecd0cf3120e33fc541..615e19cca6bb7825da6077aec96277ac4e085e62 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -710,6 +710,8 @@ class CC_EXPORT LayerTreeHostImpl
   void ImageDecodeFinished(const base::Callback<void(bool)>& embedder_callback,
                            bool decode_succeeded);
 
+  void UpdateScrollSourceInfo(bool is_wheel_scroll);
+
   using UIResourceMap = std::unordered_map<UIResourceId, UIResourceData>;
   UIResourceMap ui_resource_map_;
 
@@ -854,6 +856,10 @@ class CC_EXPORT LayerTreeHostImpl
   // begin main frame. These callbacks must only be called on the main thread.
   std::vector<base::Closure> completed_image_decode_callbacks_;
 
+  // This is used to transfer usage of touch and wheel scrolls to the main
+  // thread.
+  ScrollSourceInfo scroll_source_info_;
+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };