Implement scroll handler latency tracking

ui/events/latency_info.h

Index: ui/events/latency_info.h
diff --git a/ui/events/latency_info.h b/ui/events/latency_info.h
index f03c7f53affc8f7614516b389c53e649c1489071..92d928d615837d1323f07112b9244c135af4e67d 100644
--- a/ui/events/latency_info.h
+++ b/ui/events/latency_info.h
@@ -22,6 +22,8 @@ enum LatencyComponentType {
   INPUT_EVENT_LATENCY_BEGIN_RWH_COMPONENT,
   // Timestamp when the input event is received in plugin.
   INPUT_EVENT_LATENCY_BEGIN_PLUGIN_COMPONENT,
+  // Timestamp when a scroll update for the main thread is begun.
+  INPUT_EVENT_LATENCY_BEGIN_SCROLL_UPDATE_MAIN_COMPONENT,
   // ---------------------------NORMAL COMPONENT-------------------------------
   // Timestamp when the scroll update gesture event is sent from RWH to
   // renderer. In Aura, touch event's LatencyInfo is carried over to the gesture
@@ -39,6 +41,8 @@ enum LatencyComponentType {
   // This is special component indicating there is rendering scheduled for
   // the event associated with this LatencyInfo.
   INPUT_EVENT_LATENCY_RENDERING_SCHEDULED_COMPONENT,
+  // Timestamp when a scroll update is forwarded to the main thread.
+  INPUT_EVENT_LATENCY_FORWARD_SCROLL_UPDATE_TO_MAIN_COMPONENT,
   // Timestamp when the touch event is acked.
   INPUT_EVENT_LATENCY_ACKED_TOUCH_COMPONENT,
   // Frame number when a window snapshot was requested. The snapshot