content: Update the include paths of base/string_piece.h to its new location.

content/shell/shell.h

 // 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.
 #ifndef CONTENT_SHELL_SHELL_H_
 #define CONTENT_SHELL_SHELL_H_
 
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
-#include "base/string_piece.h"
+#include "base/strings/string_piece.h"
+#include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
-#include "content/public/browser/notification_observer.h"
 #include "content/public/browser/web_contents_delegate.h"
 #include "ipc/ipc_channel.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/size.h"
 
 #if defined(TOOLKIT_GTK)
 #include <gtk/gtk.h>
 #include "ui/base/gtk/gtk_signal.h"
 
 typedef struct _GtkToolItem GtkToolItem;
@@ skipping 234 matching lines @@
   static base::Callback<void(Shell*)> shell_created_callback_;
 
   // True if the destructur of Shell should post a quit closure on the current
   // message loop if the destructed Shell object was the last one.
   static bool quit_message_loop_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_SHELL_SHELL_H_