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

components/mus/public/interfaces/window_tree_host.mojom

 // 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.
 
 module mus.mojom;
 
 import "components/mus/public/interfaces/input_event_constants.mojom";
 import "components/mus/public/interfaces/input_event_matcher.mojom";
 import "components/mus/public/interfaces/input_events.mojom";
 import "components/mus/public/interfaces/input_key_codes.mojom";
 import "components/mus/public/interfaces/window_manager.mojom";
 import "components/mus/public/interfaces/window_tree.mojom";
 import "ui/mojo/geometry/geometry.mojom";
 
 // WindowTreeHost encapsulates a unique underlying platform window, with a tree
 // of windows.
 interface WindowTreeHost {
   // Sets the size of the platform window.
   SetSize(mojo.Size size);
 
   // Sets a title string to be displayed on the platform window.
   SetTitle(string title);
 
   // Add and remove accelerators. When accelerators are registered the
   // WindowTreeHostClient receives the event via OnAccelerator() rather than the
   // target window. The id is defined by the client and can be used to more
-  // easily identify the accelerator's action.
-  AddAccelerator(uint32 id, EventMatcher matcher);
+  // easily identify the accelerator's action. If an accelerator with the same
+  // id or the same matcher already exists, then the accelerator is not added,
+  // and returns false.
+  AddAccelerator(uint32 id, EventMatcher matcher) => (bool success);
   RemoveAccelerator(uint32 id);
 
   // Enables (or disables) child windows of |window_id| to be activated.
   AddActivationParent(uint32 window_id);
   RemoveActivationParent(uint32 window_id);
 
   ActivateNextWindow();
 
   // Sets the underlay surface offset for the specified window and additional
   // hit area. The underlay surface is drawn at the bounds of the window minus
@@ skipping 11 matching lines @@
   // An accelerator registered via AddAccelerator() has been triggered.
   OnAccelerator(uint32 id, Event event);
 };
 
 interface WindowTreeHostFactory {
   CreateWindowTreeHost(WindowTreeHost& window_tree_host,
                        WindowTreeHostClient? host_client,
                        WindowTreeClient tree_client,
                        WindowManager? window_manager);
 };