Hook up Chrome's wallpaper picker for mash.

ash/desktop_background/desktop_background_controller.cc

Index: ash/desktop_background/desktop_background_controller.cc
diff --git a/ash/desktop_background/desktop_background_controller.cc b/ash/desktop_background/desktop_background_controller.cc
index a40974dcaf8174b084f175d032da0f561aa01099..617cf9bdc7062745b797ce7f93520bc5c1620914 100644
--- a/ash/desktop_background/desktop_background_controller.cc
+++ b/ash/desktop_background/desktop_background_controller.cc
@@ -165,13 +165,18 @@ gfx::Size DesktopBackgroundController::GetMaxDisplaySizeInNative() {
   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.

[James Cook, 2016/05/19 22:29:59]

So this will fail on high DPI, because iter->size() is DIPs? If so, might be
worth filing a tracking bug for this, and reference it here, since we'll dogfood
on high DPI devices.

[msw, 2016/05/20 19:53:05]

Done.

+    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());