Add ScrollBegin.Touch/Wheel Event to UKM

ui/latency/latency_tracker.cc

Index: ui/latency/latency_tracker.cc
diff --git a/ui/latency/latency_tracker.cc b/ui/latency/latency_tracker.cc
index 494a89cc4b26957bdaeaffea6ba93763d714891a..5be7a26c79eecf59515e05f9245374a3906a8fae 100644
--- a/ui/latency/latency_tracker.cc
+++ b/ui/latency/latency_tracker.cc
@@ -159,6 +159,10 @@ void LatencyTracker::ComputeEndToEndLatencyHistograms(
                                   ".TimeToScrollUpdateSwapBegin2",
                               original_component, gpu_swap_begin_component);
 
+    ReportUkmScrollLatency("Event.ScrollBegin." + input_modality,
+                           "TimeToScrollUpdateSwapBegin", original_component,

[tdresser, 2017/06/09 15:36:36]

TimeToScrollUpdateSwapBegin -> ".TimeToScrollUpdateSwapBegin", for consistency.

[eirage, 2017/06/09 18:54:05]

I don't think it should be a '.' before the UKM metric name. Other UMA metrics
don't have it either.

[tdresser, 2017/06/09 18:55:34]

Sorry, you're right. I momentarily forgot how UKM structures things differently.

+                           gpu_swap_begin_component);
+
     // TODO(lanwei): Will remove them when M56 is stable, see
     // https://crbug.com/669618.
     UMA_HISTOGRAM_INPUT_LATENCY_HIGH_RESOLUTION_MICROSECONDS(
@@ -179,6 +183,7 @@ void LatencyTracker::ComputeEndToEndLatencyHistograms(
       ReportRapporScrollLatency(
           "Event.Latency.ScrollUpdate.Touch.TimeToScrollUpdateSwapBegin2",
           original_component, gpu_swap_begin_component);
+
       ReportUkmScrollLatency("Event.ScrollUpdate.Touch",
                              "TimeToScrollUpdateSwapBegin", original_component,
                              gpu_swap_begin_component);