Issue 5616 - Pass runAt=document_start to tabs.insertCSS

lib/cssInjection.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 21 matching lines @@
 {
   let code = selectors.join(", ") + "{display: none !important;}";
 
   try
   {
     chrome.tabs.insertCSS(tabId,
       {
         code,
         cssOrigin: "user",
         frameId,
-        matchAboutBlank: true
+        matchAboutBlank: true,
+        runAt: "document_start"
       }
     );
     return true;
   }
   catch (error)
   {
     if (/\bError processing cssOrigin\b/.test(error.message) == -1)
       throw error;
 
     userStylesheetsSupported = false;
@@ skipping 31 matching lines @@
     return {trace: false, inject: false};
   }
 
   return {selectors, trace, inject: true};
 });
 
 port.on("elemhide.injectSelectors", (msg, sender) =>
 {
   return hideElements(sender.page.id, sender.frame.id, msg.selectors);
 });