Implement scroll handler latency tracking

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 96e580fc16a4cffdb186cefdf091cec3047fabe7..09449dd64ccede6c578b5f6d0c33b6b640ec4742 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -316,6 +316,8 @@ class CC_EXPORT LayerTreeHostImpl
   bool scroll_affects_scroll_handler() const {
     return scroll_affects_scroll_handler_;
   }
+  void QueueSwapPromiseForMainThreadScrollUpdate(
+      scoped_ptr<SwapPromise> swap_promise);
 
   bool IsCurrentlyScrolling() const;
 
@@ -549,6 +551,9 @@ class CC_EXPORT LayerTreeHostImpl
   void MarkUIResourceNotEvicted(UIResourceId uid);
 
   void NotifySwapPromiseMonitorsOfSetNeedsRedraw();
+  void NotifySwapPromiseMonitorsOfForwardingToMainThread();
+  void BreakSwapPromisesForMainThreadScrollUpdate(
+      SwapPromise::DidNotSwapReason reason);
 
   typedef base::hash_map<UIResourceId, UIResourceData>
       UIResourceMap;
@@ -595,6 +600,7 @@ class CC_EXPORT LayerTreeHostImpl
   bool wheel_scrolling_;
   bool scroll_affects_scroll_handler_;
   int scroll_layer_id_when_mouse_over_scrollbar_;
+  ScopedPtrVector<SwapPromise> swap_promises_for_main_thread_scroll_update_;
 
   bool tile_priorities_dirty_;