ui: Forward declare DefaultSingletonTraits in header files.

ui/gfx/gtk_native_view_id_manager.h

 // Copyright (c) 2011 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.
 
 #ifndef UI_GFX_GTK_NATIVE_VIEW_ID_MANAGER_H_
 #define UI_GFX_GTK_NATIVE_VIEW_ID_MANAGER_H_
 
 #include <map>
 
-#include "base/memory/singleton.h"
 #include "base/synchronization/lock.h"
 #include "ui/base/ui_export.h"
 #include "ui/gfx/native_widget_types.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 typedef unsigned long XID;
 struct _GtkPreserveWindow;
 
 // NativeViewIds are the opaque values which the renderer holds as a reference
 // to a window.
 //
 // We could make NativeViewIds be the X id of the window. However, at the
 // time when we need to tell the renderer about its NativeViewId, an XID isn't
 // availible and it goes very much against the grain of the code to make it so.
 // Also, we worry that GTK might choose to change the underlying X window id
@@ skipping 103 matching lines @@
   //
   // An XID will map to NULL, if there is an outstanding reference but the
   // widget was destroyed. In this case, the destruction of the X window
   // is deferred to the dropping of all references.
   std::map<XID, PermanentXIDInfo> perm_xid_to_info_;
 
   DISALLOW_COPY_AND_ASSIGN(GtkNativeViewManager);
 };
 
 #endif  // UI_GFX_GTK_NATIVE_VIEW_ID_MANAGER_H_