Fix crash when DecrementCapturerCount() makes a delayed WasHidden() call during WebContentsImpl dest

chrome/browser/thumbnails/thumbnail_tab_helper.cc

 // 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.
 
 #include "chrome/browser/thumbnails/thumbnail_tab_helper.h"
 
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/thumbnails/render_widget_snapshot_taker.h"
 #include "chrome/browser/thumbnails/thumbnail_service.h"
@@ skipping 190 matching lines @@
       this,
       content::NOTIFICATION_RENDER_VIEW_HOST_DELETED,
       content::Source<RenderViewHost>(renderer));
 }
 
 void ThumbnailTabHelper::UpdateThumbnailIfNecessary(
     WebContents* web_contents) {
   // Destroying a WebContents may trigger it to be hidden, prompting a snapshot
   // which would be unwise to attempt <http://crbug.com/130097>. If the
   // WebContents is in the middle of destruction, do not risk it.
-  if (web_contents->IsBeingDestroyed())
+  if (!web_contents || web_contents->IsBeingDestroyed())
     return;
   // Skip if a pending entry exists. WidgetHidden can be called while navigating
   // pages and this is not a time when thumbnails should be generated.
   if (web_contents->GetController().GetPendingEntry())
     return;
   const GURL& url = web_contents->GetURL();
   Profile* profile =
       Profile::FromBrowserContext(web_contents->GetBrowserContext());
 
   scoped_refptr<thumbnails::ThumbnailService> thumbnail_service =
@@ skipping 18 matching lines @@
 void ThumbnailTabHelper::DidStartLoading(
     content::RenderViewHost* render_view_host) {
   load_interrupted_ = false;
 }
 
 void ThumbnailTabHelper::StopNavigation() {
   // This function gets called when the page loading is interrupted by the
   // stop button.
   load_interrupted_ = true;
 }