Switch ExternalProtocolObserver to use WebContentsUserData.

chrome/browser/external_protocol/external_protocol_observer.cc

 // 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.
 
 #include "chrome/browser/external_protocol/external_protocol_observer.h"
 
 #include "chrome/browser/external_protocol/external_protocol_handler.h"
 
 using content::WebContents;
 
+int ExternalProtocolObserver::kUserDataKey;
+
 ExternalProtocolObserver::ExternalProtocolObserver(WebContents* web_contents)
     : content::WebContentsObserver(web_contents) {
 }
 
 ExternalProtocolObserver::~ExternalProtocolObserver() {
 }
 
 void ExternalProtocolObserver::DidGetUserGesture() {
   ExternalProtocolHandler::PermitLaunchUrl();
 }