Delete PrintingUIWebContentsObserver on UI thread.

chrome/browser/printing/printing_ui_web_contents_observer.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.
 
 #ifndef CHROME_BROWSER_PRINTING_PRINTING_UI_WEB_CONTENTS_OBSERVER_H_
 #define CHROME_BROWSER_PRINTING_PRINTING_UI_WEB_CONTENTS_OBSERVER_H_
 
 #include "base/basictypes.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "ui/gfx/native_widget_types.h"
 
 // Wrapper used to keep track of the lifetime of a WebContents.
 // Lives on the UI thread.
 class PrintingUIWebContentsObserver : public content::WebContentsObserver {
  public:
   explicit PrintingUIWebContentsObserver(content::WebContents* web_contents);
+  virtual ~PrintingUIWebContentsObserver();
 
   // Return the parent NativeView of the observed WebContents.
   gfx::NativeView GetParentView();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(PrintingUIWebContentsObserver);
 };
 
 #endif  // CHROME_BROWSER_PRINTING_PRINTING_UI_WEB_CONTENTS_OBSERVER_H_