DevTools: disable CSS agent on disconnect.

Source/devtools/front_end/CSSStyleModel.js

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 441 matching lines @@
         this.dispatchEventToListeners(WebInspector.CSSStyleModel.Events.StyleSheetAdded, styleSheetHeader);
     },
 
     /**
      * @param {!CSSAgent.StyleSheetId} id
      */
     _styleSheetRemoved: function(id)
     {
         var header = this._styleSheetIdToHeader[id];
         console.assert(header);
+        if (!header)
+            return;
         delete this._styleSheetIdToHeader[id];
         var url = header.resourceURL();
         var frameIdToStyleSheetIds = this._styleSheetIdsForURL[url];
         frameIdToStyleSheetIds[header.frameId].remove(id);
         if (!frameIdToStyleSheetIds[header.frameId].length) {
             delete frameIdToStyleSheetIds[header.frameId];
             if (!Object.keys(this._styleSheetIdsForURL[url]).length)
                 delete this._styleSheetIdsForURL[url];
         }
         this.dispatchEventToListeners(WebInspector.CSSStyleModel.Events.StyleSheetRemoved, header);
@@ skipping 1134 matching lines @@
             for (var i = 0; i < callbacks.length; ++i)
                 callbacks[i](computedStyle);
         }
     }
 }
 
 /**
  * @type {!WebInspector.CSSStyleModel}
  */
 WebInspector.cssModel;