Remove the ability of webpages to specify strings for the onbeforeunload dialog.

content/common/frame_messages.h

 // Copyright 2013 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.
 
 // IPC messages for interacting with frames.
 // Multiply-included message file, hence no include guard.
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 1238 matching lines @@
                            base::string16     /* in - alert message */,
                            base::string16     /* in - default prompt */,
                            GURL               /* in - originating page URL */,
                            content::JavaScriptMessageType /* in - type */,
                            bool               /* out - success */,
                            base::string16     /* out - user_input field */)
 
 // Displays a dialog to confirm that the user wants to navigate away from the
 // page. Replies true if yes, and false otherwise. The reply string is ignored,
 // but is included so that we can use OnJavaScriptMessageBoxClosed.
-IPC_SYNC_MESSAGE_ROUTED3_2(FrameHostMsg_RunBeforeUnloadConfirm,
+IPC_SYNC_MESSAGE_ROUTED2_2(FrameHostMsg_RunBeforeUnloadConfirm,
                            GURL,           /* in - originating frame URL */
-                           base::string16  /* in - alert message */,
                            bool            /* in - is a reload */,
                            bool            /* out - success */,
                            base::string16  /* out - This is ignored.*/)
 
 // Asks the browser to open the color chooser.
 IPC_MESSAGE_ROUTED3(FrameHostMsg_OpenColorChooser,
                     int /* id */,
                     SkColor /* color */,
                     std::vector<content::ColorSuggestion> /* suggestions */)
 
@@ skipping 165 matching lines @@
 IPC_MESSAGE_ROUTED3(FrameHostMsg_FindMatchRects_Reply,
                     int /* version */,
                     std::vector<gfx::RectF> /* rects */,
                     gfx::RectF /* active_rect */)
 #endif
 
 // Adding a new message? Stick to the sort order above: first platform
 // independent FrameMsg, then ifdefs for platform specific FrameMsg, then
 // platform independent FrameHostMsg, then ifdefs for platform specific
 // FrameHostMsg.