Adds ability for pre-target accelerators to not consume events

ash/mus/window_manager.cc

 // 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.
 
 #include "ash/mus/window_manager.h"
 
 #include <stdint.h>
 
 #include <utility>
 
@@ skipping 211 matching lines @@
     bool janky) {
   for (auto* window : client_windows)
     SetWindowIsJanky(window, janky);
 }
 
 void WindowManager::OnWmNewDisplay(::ui::Window* window,
                                    const display::Display& display) {
   CreateRootWindowController(window, display);
 }
 
-void WindowManager::OnAccelerator(uint32_t id, const ui::Event& event) {
+ui::mojom::EventResult WindowManager::OnAccelerator(uint32_t id,
+                                                    const ui::Event& event) {
   switch (id) {
     case kWindowSwitchAccelerator:
       window_manager_client()->ActivateNextWindow();
       break;
     default:
       FOR_EACH_OBSERVER(WindowManagerObserver, observers_,
                         OnAccelerator(id, event));
       break;
   }
+  return ui::mojom::EventResult::HANDLED;
 }
 
 }  // namespace mus
 }  // namespace ash