Add Keyboard Latency UMA Metrics.

ui/events/event.cc

Index: ui/events/event.cc
diff --git a/ui/events/event.cc b/ui/events/event.cc
index ce99ff82d68ee08dd10cdfe51f89c3bedfcfc56a..2e535a0640e62829dcafcf98f83bce3d3e9e9059 100644
--- a/ui/events/event.cc
+++ b/ui/events/event.cc
@@ -149,13 +149,18 @@ SourceEventType EventTypeToLatencySourceEventType(EventType type) {
     case ET_SCROLL_FLING_CANCEL:
       return SourceEventType::UNKNOWN;
 
+    case ui::ET_KEY_PRESSED:
+      return ui::SourceEventType::KEY;
+
     case ET_MOUSE_PRESSED:
     case ET_MOUSE_DRAGGED:
     case ET_MOUSE_RELEASED:
     case ET_MOUSE_MOVED:
     case ET_MOUSE_ENTERED:
     case ET_MOUSE_EXITED:
-    case ET_KEY_PRESSED:
+    // We measure latency for key presses, not key releases. Most behavior is

[mfomitchev, 2017/05/04 20:36:05]

Looking closer at this, I think the way this works in problematic. 

1. It's a little odd to define the type of the event based on whether we measure
latency for it or not.
2. We actually do measure latency regardless of type, e.g. these latencies would
appear in a traces. We just don't report some UMA metrics if the type is OTHER.
3.If the event goes through RWH::ForwardKeyboardEventWithLatencyInfo (e.g. in
RenderWidgetHostViewGuest::OnHandleInputEvent), it seems like will get the KEY
type event if it is a release event.

Perhaps we should rename the type to KEY_PRESSED to address (1), add checks for
WebInputEvent type to address (3), and clarify the comment to address (2)?

[tdresser, 2017/05/10 14:27:16]

Done - renamed to KEY_PRESS, for consistency with other source event types.

+    // keyed off of presses, and release latency is higher than press latency as
+    // it's impacted by event handling of the press event.
     case ET_KEY_RELEASED:
     case ET_MOUSE_CAPTURE_CHANGED:
     case ET_DROP_TARGET_EVENT:
@@ -1141,6 +1146,11 @@ KeyEvent::KeyEvent(const base::NativeEvent& native_event, int event_flags)
       key_code_(KeyboardCodeFromNative(native_event)),
       code_(CodeFromNative(native_event)),
       is_char_(IsCharFromNative(native_event)) {
+  latency()->AddLatencyNumberWithTimestamp(
+      INPUT_EVENT_LATENCY_ORIGINAL_COMPONENT, 0, 0,
+      base::TimeTicks::FromInternalValue(time_stamp().ToInternalValue()), 1);
+  latency()->AddLatencyNumber(INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
+
   if (IsRepeated(*this))
     set_flags(flags() | ui::EF_IS_REPEAT);