mus: Notify the client whether adding an accelerator was successful.

components/mus/ws/event_dispatcher.h

 // Copyright 2015 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.
 
 #ifndef COMPONENTS_MUS_WS_EVENT_DISPATCHER_H_
 #define COMPONENTS_MUS_WS_EVENT_DISPATCHER_H_
 
 #include <map>
 
 #include "base/memory/scoped_ptr.h"
@@ skipping 15 matching lines @@
 // Handles dispatching events to the right location as well as updating focus.
 class EventDispatcher : public ServerWindowObserver {
  public:
   explicit EventDispatcher(EventDispatcherDelegate* delegate);
   ~EventDispatcher() override;
 
   void set_root(ServerWindow* root) { root_ = root; }
 
   void set_surface_id(cc::SurfaceId surface_id) { surface_id_ = surface_id; }
 
-  void AddAccelerator(uint32_t id, mojom::EventMatcherPtr event_matcher);
+  // Adds an accelerator with the given id and event-matcher. If an accelerator
+  // already exists with the same id or the same matcher, then the accelerator
+  // is not added. Returns whether adding the accelerator was successful or not.
+  bool AddAccelerator(uint32_t id, mojom::EventMatcherPtr event_matcher);
   void RemoveAccelerator(uint32_t id);
 
   void OnEvent(mojom::EventPtr event);
 
  private:
   // Keeps track of state associated with a pointer down until the
   // corresponding up/cancel.
   struct PointerTarget {
     PointerTarget() : window(nullptr), in_nonclient_area(false) {}
 
@@ skipping 53 matching lines @@
   using PointerIdToTargetMap = std::map<int32_t, PointerTarget>;
   PointerIdToTargetMap pointer_targets_;
 
   DISALLOW_COPY_AND_ASSIGN(EventDispatcher);
 };
 
 }  // namespace ws
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_WS_EVENT_DISPATCHER_H_