Use mojo Shelf interfaces for both mash and classic ash.

ash/display/window_tree_host_manager.cc

 // Copyright (c) 2012 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/display/window_tree_host_manager.h"
 
 #include <algorithm>
 #include <cmath>
 #include <map>
 #include <memory>
@@ skipping 314 matching lines @@
   CHECK_NE(display::Display::kInvalidDisplayID, primary_display_id);
   return primary_display_id;
 }
 
 aura::Window* WindowTreeHostManager::GetPrimaryRootWindow() {
   return GetRootWindowForDisplayId(primary_display_id);
 }
 
 aura::Window* WindowTreeHostManager::GetRootWindowForDisplayId(int64_t id) {
   AshWindowTreeHost* host = GetAshWindowTreeHostForDisplayId(id);
-  CHECK(host);
-  return GetWindow(host);
+  return host ? GetWindow(host) : nullptr;
 }
 
 AshWindowTreeHost* WindowTreeHostManager::GetAshWindowTreeHostForDisplayId(
     int64_t display_id) {
-  CHECK_EQ(1u, window_tree_hosts_.count(display_id)) << "display id = "
-                                                     << display_id;
+  if (window_tree_hosts_.count(display_id) == 0)

[sky, 2016/10/11 02:40:37]

use find and a conditional to avoid the double lookup.

[msw, 2016/10/11 15:38:00]

Done.

+    return nullptr;
   return window_tree_hosts_[display_id];
 }
 
 void WindowTreeHostManager::CloseChildWindows() {
   for (WindowTreeHostMap::const_iterator it = window_tree_hosts_.begin();
        it != window_tree_hosts_.end(); ++it) {
     aura::Window* root_window = GetWindow(it->second);
     RootWindowController* controller = GetRootWindowController(root_window);
     if (controller) {
       controller->CloseChildWindows();
@@ skipping 509 matching lines @@
   SetDisplayPropertiesOnHost(ash_host, display);
 
 #if defined(OS_CHROMEOS)
   if (switches::ConstrainPointerToRoot())
     ash_host->ConfineCursorToRootWindow();
 #endif
   return ash_host;
 }
 
 }  // namespace ash