Hook up Chrome's wallpaper picker for mash.

ash/desktop_background/desktop_background_controller.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/desktop_background/desktop_background_controller.h"
 
 #include "ash/ash_switches.h"
 #include "ash/desktop_background/desktop_background_controller_observer.h"
 #include "ash/desktop_background/desktop_background_view.h"
 #include "ash/desktop_background/desktop_background_widget_controller.h"
@@ skipping 147 matching lines @@
 
   InstallDesktopController(root_window);
 }
 
 // static
 gfx::Size DesktopBackgroundController::GetMaxDisplaySizeInNative() {
   int width = 0;
   int height = 0;
   std::vector<display::Display> displays =
       display::Screen::GetScreen()->GetAllDisplays();
-  DisplayManager* display_manager = Shell::GetInstance()->display_manager();
+  DisplayManager* display_manager =
+      Shell::HasInstance() ? Shell::GetInstance()->display_manager() : nullptr;
 
   for (std::vector<display::Display>::iterator iter = displays.begin();
        iter != displays.end(); ++iter) {
     // Don't use size_in_pixel because we want to use the native pixel size.
-    gfx::Size size_in_pixel =
-        display_manager->GetDisplayInfo(iter->id()).bounds_in_native().size();
+    // TODO(msw): Fix this for Mash/Mus.
+    gfx::Size size_in_pixel = display_manager
+                                  ? display_manager->GetDisplayInfo(iter->id())
+                                        .bounds_in_native()
+                                        .size()
+                                  : iter->size();
     if (iter->rotation() == display::Display::ROTATE_90 ||
         iter->rotation() == display::Display::ROTATE_270) {
       size_in_pixel = gfx::Size(size_in_pixel.height(), size_in_pixel.width());
     }
     width = std::max(size_in_pixel.width(), width);
     height = std::max(size_in_pixel.height(), height);
   }
   return gfx::Size(width, height);
 }
 
@@ skipping 91 matching lines @@
                 : kShellWindowId_DesktopBackgroundContainer;
 }
 
 void DesktopBackgroundController::UpdateWallpaper(bool clear_cache) {
   current_wallpaper_.reset(NULL);
   ash::Shell::GetInstance()->user_wallpaper_delegate()->UpdateWallpaper(
       clear_cache);
 }
 
 }  // namespace ash