Fixes 4328 - Multiple shadowRoot deprecated warnings

include.preload.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 441 matching lines @@
                                                addElemHideSelectors);
 
   // Use Shadow DOM if available to don't mess with web pages that rely on
   // the order of their own <style> tags (#309).
   //
   // However, creating a shadow root breaks running CSS transitions. So we
   // have to create the shadow root before transistions might start (#452).
   //
   // Also, using shadow DOM causes issues on some Google websites,
   // including Google Docs, Gmail and Blogger (#1770, #2602, #2687).
+  //
+  // Finally since some users have both AdBlock and Adblock Plus installed we
+  // have to consider how the two extensions interact. For example we want to
+  // avoid creating the shadowRoot twice.
   if ("createShadowRoot" in document.documentElement &&
       !/\.(?:google|blogger)\.com$/.test(document.domain))
   {
-    shadow = document.documentElement.createShadowRoot();
+    shadow = document.documentElement.shadowRoot ||
+             document.documentElement.createShadowRoot();
     shadow.appendChild(document.createElement("shadow"));
 
     // Stop the website from messing with our shadowRoot
     if ("shadowRoot" in Element.prototype)
     {
       runInDocument(document, function()
       {
         var ourShadowRoot = document.documentElement.shadowRoot;
+        if (!ourShadowRoot)
+          return;
         var desc = Object.getOwnPropertyDescriptor(Element.prototype, "shadowRoot");
         var shadowRoot = Function.prototype.call.bind(desc.get);
 
         Object.defineProperty(Element.prototype, "shadowRoot", {
           configurable: true, enumerable: true, get: function()
           {
             var shadow = shadowRoot(this);
             return shadow == ourShadowRoot ? null : shadow;
           }
         });
@@ skipping 127 matching lines @@
   }, true);
 
   return updateStylesheet;
 }
 
 if (document instanceof HTMLDocument)
 {
   checkSitekey();
   window.updateStylesheet = init(document);
 }