Add UMA/Telemetry stats for end-to-end scroll latency

ui/base/latency_info.h

Index: ui/base/latency_info.h
diff --git a/ui/base/latency_info.h b/ui/base/latency_info.h
index 423182a0379ea382d9b40cda4c231511904750ff..4ddebb7524e3cd73e1aec398fe772d8e1db3a4d0 100644
--- a/ui/base/latency_info.h
+++ b/ui/base/latency_info.h
@@ -17,14 +17,13 @@ namespace ui {
 enum LatencyComponentType {
   // Timestamp when the input event is sent from RenderWidgetHost to renderer.
   INPUT_EVENT_LATENCY_RWH_COMPONENT,
-  // Timestamp when the injected event is sent from RenderWidgetHost to
-  // renderer. For example, in Aura, touch event's LatencyInfo is carried over
-  // to the injected gesture event. So gesture event's
-  // INPUT_EVENT_LATENCY_RWH_COMPONENT is the timestamp when its original touch
-  // events is sent from RWH to renderer. In non-aura platform,
-  // INPUT_EVENT_LATENCY_RWH_COMPONENT is the same as
-  // INPUT_EVENT_LATENCY_INJECTED_RWH_COMPONENT.
-  INPUT_EVENT_LATENCY_INJECTED_RWH_COMPONENT,
+  // Timestamp when the scroll gesture event is sent from RenderWidgetHost to
+  // renderer. In Aura, touch event's LatencyInfo is carried over to the gesture
+  // event. So gesture event's INPUT_EVENT_LATENCY_RWH_COMPONENT is the
+  // timestamp when its original touch events is sent from RWH to renderer.
+  // In non-aura platform, INPUT_EVENT_LATENCY_GESTURE_SCROLL_RWH_COMPONENT
+  // is the same as INPUT_EVENT_LATENCY_RWH_COMPONENT.
+  INPUT_EVENT_LATENCY_GESTURE_SCROLL_RWH_COMPONENT,

[Rick Byers, 2013/07/10 17:27:14]

By the way, I'm guessing you'll eventually want to track GestureScrollStart and
GestureScrollUpdate separately since you'll be fixing the performance of
GestureScrollUpdate but not GestureScrollStart.

Eg. if a particular gesture has say a 1000ms latency for start, and then 100
updates each with 10ms latency, reporting that as an average of ~20ms per event
(or even a histogram with 99% <=10ms) hides the problem of a 1s start delay.

But doing that in a later CL makes sense.

[Yufeng Shen (Slow to review), 2013/07/10 19:38:24]

right, changed to add SCROLL_UPDATE_RWH_COMPONENT only to GestureScrollUpdate 

   // Original timestamp for input event (e.g. timestamp from kernel).
   INPUT_EVENT_LATENCY_ORIGINAL_COMPONENT,
   // Timestamp when the UI event is created.