Issue 4494 - Avoid causing some sandbox related warnings

inject.preload.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2017 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 369 matching lines @@
   RealRTCPeerConnection.prototype.constructor = boundWrappedRTCPeerConnection;
 
   if ("RTCPeerConnection" in window)
     window.RTCPeerConnection = boundWrappedRTCPeerConnection;
   if ("webkitRTCPeerConnection" in window)
     window.webkitRTCPeerConnection = boundWrappedRTCPeerConnection;
 }
 
 if (document instanceof HTMLDocument)
 {
-  let script = document.createElement("script");
-  script.type = "application/javascript";
-  script.async = false;
-  script.textContent = "(" + injected + ")('" + randomEventName + "');";
-  document.documentElement.appendChild(script);
-  document.documentElement.removeChild(script);
+  let sandbox = window.frameElement &&
+                window.frameElement.getAttribute("sandbox");
+
+  if (typeof sandbox != "string" || /(^|\s)allow-scripts(\s|$)/i.test(sandbox))
+  {
+    let script = document.createElement("script");
+    script.type = "application/javascript";
+    script.async = false;
+    script.textContent = "(" + injected + ")('" + randomEventName + "');";
+    document.documentElement.appendChild(script);
+    document.documentElement.removeChild(script);
+  }
 }