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 51b9b4f2e33d31a0663ff35821a4b8ac0f21f482..264dadc89fc114aefd227a5f46117467d28338ab 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"
@@ -1014,6 +1015,22 @@ 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];
+    if (!latency_info.FindLatency(

[Yufeng Shen (Slow to review), 2014/07/04 18:34:34]

why do we need this check ?

[Sami, 2014/07/07 14:28:18]

Ah, you're right, the latency infos can only flow though here once so there's no
need for this check.

+            ui::INPUT_EVENT_LATENCY_SCROLL_UPDATE_BLINK_COMPONENT, 0, 0)) {
+      latency_info.AddLatencyNumber(
+          ui::INPUT_EVENT_LATENCY_SCROLL_UPDATE_BLINK_COMPONENT, 0, 0);

[Yufeng Shen (Slow to review), 2014/07/04 18:34:34]

Are we planning to use this new component anywhere ?
If we only care about end-to-end latency, then we don't need this new component.
Or do you plan to use this component to derive any new metrics ?

[Sami, 2014/07/07 14:28:18]

Yeah, end-to-end is what really matters here. I wasn't planning to use the new
component for anything in particular, so let's just remove it.

+      TRACE_EVENT_FLOW_STEP0("input",
+                             "LatencyInfo.Flow",
+                             TRACE_ID_DONT_MANGLE(latency_info.trace_id),
+                             "Blink");
+      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;