Implement scroll handler latency tracking

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 2d7a9323edf74e7bceb7eab96100f2d8996b12f6..061a8041e62ded0b334a0ca12bb67f6d659386d3 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -503,6 +503,11 @@ LayerTreeHostImpl::CreateLatencyInfoSwapPromiseMonitor(
       new LatencyInfoSwapPromiseMonitor(latency, NULL, this));
 }
 
+void LayerTreeHostImpl::QueueSwapPromiseForMainThreadScrollUpdate(
+    scoped_ptr<SwapPromise> swap_promise) {
+  swap_promises_for_main_thread_scroll_update_.push_back(swap_promise.Pass());
+}
+
 void LayerTreeHostImpl::TrackDamageForAllSurfaces(
     LayerImpl* root_draw_layer,
     const LayerImplList& render_surface_layer_list) {
@@ -2648,6 +2653,11 @@ bool LayerTreeHostImpl::ScrollBy(const gfx::Point& viewport_point,
 
   bool did_scroll_content = did_scroll_x || did_scroll_y;
   if (did_scroll_content) {
+    // If we are scrolling with an active scroll handler, forward latency
+    // tracking information to the main thread so the delay introduced by the
+    // handler is accounted for.
+    if (scroll_affects_scroll_handler())
+      NotifySwapPromiseMonitorsOfForwardingToMainThread();
     client_->SetNeedsCommitOnImplThread();
     SetNeedsRedraw();
     client_->RenewTreePriority();
@@ -2949,6 +2959,7 @@ scoped_ptr<ScrollAndScaleSet> LayerTreeHostImpl::ProcessScrollDeltas() {
   CollectScrollDeltas(scroll_info.get(), active_tree_->root_layer());
   scroll_info->page_scale_delta = active_tree_->page_scale_delta();
   active_tree_->set_sent_page_scale_delta(scroll_info->page_scale_delta);
+  scroll_info->swap_promises.swap(swap_promises_for_main_thread_scroll_update_);
 
   return scroll_info.Pass();
 }
@@ -3362,6 +3373,12 @@ void LayerTreeHostImpl::NotifySwapPromiseMonitorsOfSetNeedsRedraw() {
     (*it)->OnSetNeedsRedrawOnImpl();
 }
 
+void LayerTreeHostImpl::NotifySwapPromiseMonitorsOfForwardingToMainThread() {
+  std::set<SwapPromiseMonitor*>::iterator it = swap_promise_monitor_.begin();
+  for (; it != swap_promise_monitor_.end(); it++)
+    (*it)->OnForwardScrollUpdateToMainThreadOnImpl();
+}
+
 void LayerTreeHostImpl::RegisterPictureLayerImpl(PictureLayerImpl* layer) {
   DCHECK(std::find(picture_layers_.begin(), picture_layers_.end(), layer) ==
          picture_layers_.end());