Hook up Chrome's wallpaper picker for mash.

ash/mus/shell_delegate_mus.cc

Index: ash/mus/shell_delegate_mus.cc
diff --git a/ash/mus/shell_delegate_mus.cc b/ash/mus/shell_delegate_mus.cc
index 716b5d10876b1f4f32f8c2aebb101b2a2d2c614e..52ea1bb2dd2a0d8c68a9c8c309b222643ef6127c 100644
--- a/ash/mus/shell_delegate_mus.cc
+++ b/ash/mus/shell_delegate_mus.cc
@@ -5,7 +5,6 @@
 #include "ash/mus/shell_delegate_mus.h"
 
 #include "ash/default_accessibility_delegate.h"
-#include "ash/default_user_wallpaper_delegate.h"
 #include "ash/gpu_support_stub.h"
 #include "ash/media_delegate.h"
 #include "ash/mus/app_list_presenter_mus.h"
@@ -13,6 +12,7 @@
 #include "ash/mus/context_menu_mus.h"
 #include "ash/mus/pointer_watcher_delegate_mus.h"
 #include "ash/mus/shelf_delegate_mus.h"
+#include "ash/mus/user_wallpaper_delegate_mus.h"
 #include "ash/session/session_state_delegate.h"
 #include "ash/system/tray/default_system_tray_delegate.h"
 #include "base/memory/ptr_util.h"
@@ -185,8 +185,7 @@ SystemTrayDelegate* ShellDelegateMus::CreateSystemTrayDelegate() {
 }
 
 UserWallpaperDelegate* ShellDelegateMus::CreateUserWallpaperDelegate() {
-  NOTIMPLEMENTED() << " Using the default UserWallpaperDelegate implementation";
-  return new DefaultUserWallpaperDelegate();
+  return new UserWallpaperDelegateMus();
 }
 
 SessionStateDelegate* ShellDelegateMus::CreateSessionStateDelegate() {