Mac: Fix crash due to occluding view during tear-down

content/browser/web_contents/web_contents_view_mac.mm

 // 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.
 
 #import <Carbon/Carbon.h>
 
 #import "content/browser/web_contents/web_contents_view_mac.h"
 
 #include <string>
 
@@ skipping 609 matching lines @@
                  name:NSWindowDidChangeOcclusionStateNotification
                object:newWindow];
     }
   }
 }
 
 - (void)windowChangedOcclusionState:(NSNotification*)notification {
   DCHECK(base::mac::IsOSMavericksOrLater());
   NSWindow* window = [notification object];
   WebContentsImpl* webContents = [self webContents];
-  if (window && webContents) {
+  if (window && webContents && !webContents->IsBeingDestroyed()) {
     if ([window occlusionState] & NSWindowOcclusionStateVisible) {
       webContents->WasUnOccluded();
     } else {
       webContents->WasOccluded();
     }
   }
 }
 
 @end