Implement scroll handler latency tracking

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 48f4864ef4a14e903aba2665fa85a989f725ea8a..cb4ea36dd5ecc1a58ab3944e0f025f764442ba49 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -18,6 +18,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "cc/animation/animation_registrar.h"
 #include "cc/animation/layer_animation_controller.h"
+#include "cc/base/latency_info_swap_promise.h"
 #include "cc/base/math_util.h"
 #include "cc/debug/devtools_instrumentation.h"
 #include "cc/debug/rendering_stats_instrumentation.h"
@@ -1021,6 +1022,17 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_layer_.get())
     return;
 
+  for (size_t i = 0; i < info.latency_infos.size(); ++i) {
+    ui::LatencyInfo latency_info = info.latency_infos[i];
+    TRACE_EVENT_FLOW_STEP0("input",
+                           "LatencyInfo.Flow",
+                           TRACE_ID_DONT_MANGLE(latency_info.trace_id),
+                           "Blink scroll update");

[danakj, 2014/07/08 16:49:07]

layering violation: cc doesn't know about blink. You can say main-thread scroll
update or something like that

[Sami, 2014/07/09 13:28:55]

Ah, sorry for the slip-up :) Fixed.

+    scoped_ptr<SwapPromise> swap_promise(
+        new LatencyInfoSwapPromise(latency_info));
+    QueueSwapPromise(swap_promise.Pass());
+  }
+
   gfx::Vector2d inner_viewport_scroll_delta;
   gfx::Vector2d outer_viewport_scroll_delta;