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. See WindowTree for details on how it handles the
+// AcceleratorPhase.
 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,
 };