Fix "Uncaught ReferenceError: ext is not defined"

This patch is temporary and can be reverted once the Chromium bug
(https://crbug.com/416907) has been fixed.

To make merging/reverting the patch easier, I have changed the main if-block
to a function that takes the original if-body as a main function. The main
function is called whenever the bootstrapper has made sure that the environment
is ready. (The alternatives, disabling the content script or using XHR, were
not so appealing.)

Fixes
https://issues.adblockplus.org/ticket/1370
https://adblockplus.org/forum/viewtopic.php?f=10&t=25507

I hereby waive any rights of this contributed piece of software, you may do whatever you want with it (this comment is an alternative to signing the CLA at https://adblockplus.org/eyeo-contributor-license-agreement.pdf).
Our coding style guidelines require double quotes, moreover I'd prefer to check `"ext" in window` instead.