Add Keyboard Latency UMA Metrics.

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 8210ae5c1985c294e5e92ed162c0f92ad95dcacf..227a941bc37474ba821ff5932c4644d9baede2a7 100644
--- a/ui/events/latency_info.h
+++ b/ui/events/latency_info.h
@@ -109,6 +109,7 @@ enum SourceEventType {
   UNKNOWN,
   WHEEL,
   TOUCH,
+  KEY,
   OTHER,
   SOURCE_EVENT_TYPE_LAST = OTHER,
 };