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 caa380ec8623f26c387539b3067298a87da5a698..12c6dda1f5f0da5160a89215d4897e61ef0241e0 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::QueueScrollUpdateLatencyInfo(
+    const ui::LatencyInfo& latency_info) {
+  scroll_update_latency_infos_.push_back(latency_info);
+}
+
 void LayerTreeHostImpl::TrackDamageForAllSurfaces(
     LayerImpl* root_draw_layer,
     const LayerImplList& render_surface_layer_list) {
@@ -2462,6 +2467,11 @@ 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();
@@ -2763,6 +2773,7 @@ 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);
+  std::swap(scroll_info->latency_infos, scroll_update_latency_infos_);
 
   return scroll_info.Pass();
 }
@@ -3139,6 +3150,12 @@ 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());