Use mojo SystemTray interfaces for both mash and classic ash

ash/mus/bridge/wm_shell_mus.cc

 // Copyright 2016 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/bridge/wm_shell_mus.h"
 
 #include <utility>
 
 #include "ash/common/accelerators/accelerator_controller.h"
 #include "ash/common/key_event_watcher.h"
@@ skipping 185 matching lines @@
     int64_t id) {
   for (WmRootWindowControllerMus* root_window_controller :
        root_window_controllers_) {
     if (root_window_controller->GetDisplay().id() == id)
       return root_window_controller;
   }
   NOTREACHED();
   return nullptr;
 }
 
+bool WmShellMus::IsRunningInMash() const {
+  return true;
+}
+
 WmWindow* WmShellMus::NewWindow(ui::wm::WindowType window_type,
                                 ui::LayerType layer_type) {
   WmWindowMus* window = WmWindowMus::Get(window_tree_client()->NewWindow());
   window->set_wm_window_type(window_type);
   // TODO(sky): support layer_type.
   NOTIMPLEMENTED();
   return window;
 }
 
 WmWindow* WmShellMus::GetFocusedWindow() {
@@ skipping 241 matching lines @@
                     OnWindowActivated(gained_active, lost_active));
 }
 
 void WmShellMus::OnDidDestroyClient(ui::WindowTreeClient* client) {
   DCHECK_EQ(window_tree_client(), client);
   client->RemoveObserver(this);
 }
 
 }  // namespace mus
 }  // namespace ash