ui: Forward declare DefaultSingletonTraits in header files.

ui/base/x/events_x.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/base/events.h"
 
 #include <string.h>
 #include <X11/extensions/XInput.h>
 #include <X11/extensions/XInput2.h>
 #include <X11/Xlib.h>
 
 #include "base/command_line.h"
 #include "base/logging.h"
+#include "base/memory/singleton.h"
 #include "base/message_pump_aurax11.h"
 #include "ui/base/keycodes/keyboard_code_conversion_x.h"
 #include "ui/base/touch/touch_factory.h"
 #include "ui/base/ui_base_switches.h"
 #include "ui/base/x/valuators.h"
 #include "ui/base/x/x11_util.h"
 #include "ui/gfx/display.h"
 #include "ui/gfx/point.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/screen.h"
@@ skipping 949 matching lines @@
     noop->xclient.format = 8;
     DCHECK(!noop->xclient.display);
   }
   // Make sure we use atom from current xdisplay, which may
   // change during the test.
   noop->xclient.message_type = GetNoopEventAtom();
   return noop;
 }
 
 }  // namespace ui