[Extensions] Fix v8 deprecated usages

"TL;DR: if you're working on code that interacts with V8, please be aware that from Thursday on, it won't be possible anymore to use APIs marked as deprecated."

http://permalink.gmane.org/gmane.comp.web.chromium.devel/47781

This patch fixes many of the deprecated usages we had. Behavior of the code should not change. This will also ease the life of future Chromium updates, since we'll inherit the restriction from Chromium build system.