Switch ZoomTabHelper to use WebContentsUserData.

chrome/browser/ui/zoom/zoom_observer.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 CHROME_BROWSER_UI_ZOOM_ZOOM_OBSERVER_H_
 #define CHROME_BROWSER_UI_ZOOM_ZOOM_OBSERVER_H_
 
-class TabContents;
+namespace content {
+class WebContents;
+}
 
 // Interface for objects that wish to be notified of changes in ZoomController.
 class ZoomObserver {
  public:
   // Notification that the zoom percentage has changed.
-  virtual void OnZoomChanged(TabContents* source, bool can_show_bubble) = 0;
+  virtual void OnZoomChanged(content::WebContents* source,
+                             bool can_show_bubble) = 0;
 
  protected:
   virtual ~ZoomObserver() {}
 };
 
 #endif  // CHROME_BROWSER_UI_ZOOM_ZOOM_OBSERVER_H_