[Extensions] Move opening an absolute test URL to another function.

Bug: 1171429
Change-Id: Ica4b5b863ca824bfa7fe152e33972ca79893a0b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2847398


Commit-Queue: David Bertoni <dbertoni@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880649}