Extensions: Simplify extension page CSP parsing.

This CL simplifies the CSP parsing logic for extension pages a bit. It should
introduce no behavior change.

BUG=914224

Change-Id: I5c1c2fbcc48364f0e013b16fa6d545da428193b1
Reviewed-on: https://chromium-review.googlesource.com/c/1391361
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619079}