Cookie store should be created and initialized on the IO thread

We currently create and initialize the cookie store on the UI thread because it's required to make the UI thread cookie store proxy immediately available to the CookieManager API. However, the cookie store really lives on and is used on the IO thread, and net::CookieStore is not thread safe. Whilst it's sort-of ok for the time being (we guarantee there aren't any concurrent accesses during initialization), this will break if Chromium modifies net::CookieMonster to assert that it's called from a single thread.