Adds ability for pre-target accelerators to not consume events

ui/events/mojo/event_constants.mojom

Index: ui/events/mojo/event_constants.mojom
diff --git a/ui/events/mojo/event_constants.mojom b/ui/events/mojo/event_constants.mojom
index 445cffbade6c7bc45755ede8f25e6898ad210201..b2c09332a14540edc074807ce7b80dfaae291d39 100644
--- a/ui/events/mojo/event_constants.mojom
+++ b/ui/events/mojo/event_constants.mojom
@@ -4,10 +4,13 @@
 
 module ui.mojom;
 
-// Denotes during which phase of event targeting that an accelerator will be
-// notified of an event. PRE_TARGET will be notified instead of the target
-// window. POST_TARGET will be notified after the target window, and only if the
-// event was not consumed by the target.
+// AcceleratorPhase specifies which phase of event matching the matcher should
+// be considered for. Event processing happens in the following order:
+// . EventMatcher registered for PRE_TARGET. If the EventMatcher does not

[dcheng, 2016/07/07 04:04:21]

I was wondering this while reviewing the dragging CL, but can a renderer somehow
use SetEventObserver to capture events it shouldn't be privy too? I'm assuming
the WindowTree that the embedded renderer sees is scoped to just the stuff under
that renderer?

[sky, 2016/07/07 13:38:38]

I've moved these comments to clarify they are for accelerators, which only the
windowmanager can register. Other clients can register an eventobserver which
can see events they normally wouldn't, but event observers can't consume events.

+//   consume the event, then the following are considered.
+// . Target window (lookup of the target window depends upon the event type).
+// . EventMatcher registered for POST_TARGET. These matchers are only considered
+//   if the target window does not consume the event.
 enum AcceleratorPhase {
   PRE_TARGET,
   POST_TARGET,