Issue 7104 - Clear style sheets when frame structure is updated

ext/background.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present 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 150 matching lines @@
     let frame = createFrame(tabId, frameId);
     frame.url = new URL(url);
 
     let frames = framesOfTabs.get(tabId);
     let parentFrame = frames.get(parentFrameId);
     if (!parentFrame && parentFrameId > 0)
       parentFrame = frames.get(0);
 
     if (parentFrame)
       frame.parent = parentFrame;
+
+    frame.injectedStyleSheets = null;
   }
 
   browser.webRequest.onHeadersReceived.addListener(details =>
   {
     // We have to update the frame structure when switching to a new
     // document, so that we process any further requests made by that
     // document in the right context. Unfortunately, we cannot rely
     // on webNavigation.onCommitted since it isn't guaranteed to fire
     // before any subresources start downloading[1]. As an
     // alternative we use webRequest.onHeadersReceived for HTTP(S)
@@ skipping 337 matching lines @@
           return frames.get(0) || null;
         }
       };
     }
 
     return ext.onMessage._dispatch(
       message, sender, sendResponse
     ).includes(true);
   });
 }