ui: Forward declare DefaultSingletonTraits in header files.

ui/views/widget/x11_desktop_handler.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 "ui/views/widget/x11_desktop_handler.h"
 
-#include "ui/views/widget/desktop_native_widget_helper_aura.h"
+#include "base/message_loop.h"
 #include "ui/aura/desktop/desktop_activation_client.h"
 #include "ui/aura/dispatcher_linux.h"
 #include "ui/aura/env.h"
 #include "ui/aura/focus_manager.h"
 #include "ui/aura/root_window.h"
 #include "ui/base/x/x11_util.h"
+#include "ui/views/widget/desktop_native_widget_helper_aura.h"
 
 namespace {
 
 const char* kAtomsToCache[] = {
   "_NET_ACTIVE_WINDOW",
   NULL
 };
 
 // Our global instance. Deleted when our Env() is deleted.
 views::X11DesktopHandler* g_handler = NULL;
@@ skipping 71 matching lines @@
   aura::RootWindow* root_window =
       aura::RootWindow::GetForAcceleratedWidget(xid);
   aura::Window* window = root_window ?
       views::DesktopNativeWidgetHelperAura::GetViewsWindowForRootWindow(
           root_window) : NULL;
 
   desktop_activation_client_->ActivateWindow(window);
 }
 
 }  // namespace views