Adds ability for pre-target accelerators to not consume events

ui/events/mojo/event_constants.mojom

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 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,
 };
 
 enum EventType {
   UNKNOWN,
   KEY_PRESSED,
   KEY_RELEASED,
   POINTER_CANCEL,
@@ skipping 34 matching lines @@
   PEN,
   TOUCH,
 };
 
 enum WheelMode {
   PIXEL,
   LINE,
   PAGE,
   SCALING,
 };