Delete PrintingUIWebContentsObserver on UI thread.

chrome/browser/printing/printing_ui_web_contents_observer.cc

 // 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.
 
 #include "chrome/browser/printing/printing_ui_web_contents_observer.h"
 
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/web_contents.h"
 
 PrintingUIWebContentsObserver::PrintingUIWebContentsObserver(
     content::WebContents* web_contents)
     : content::WebContentsObserver(web_contents) {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 }
 
+PrintingUIWebContentsObserver::~PrintingUIWebContentsObserver() {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+}
+
 gfx::NativeView PrintingUIWebContentsObserver::GetParentView() {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   return web_contents() ? web_contents()->GetNativeView() : NULL;
 }