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 115 matching lines @@
       framesOfTabs.set(tabId, frames);
     }
 
     let frame = frames.get(frameId);
     if (!frame)
     {
       frame = {};
       frames.set(frameId, frame);
     }
 
+    frame.state = Object.create(null);
+
     return frame;
   }
 
   function updatePageFrameStructure(frameId, tabId, url, parentFrameId)
   {
     if (frameId == 0)
     {
       let page = new Page({id: tabId, url});
 
       removeFromAllPageMaps(tabId);
@@ skipping 371 matching lines @@
           return frames.get(0) || null;
         }
       };
     }
 
     return ext.onMessage._dispatch(
       message, sender, sendResponse
     ).includes(true);
   });
 }