Extensions: Implement "content_security_policy.extension_pages" manifest key.

This CL implements the "content_security_policy.extension_pages" manifest key.
This is part of allowing "content_security_policy" key to be specified as a
dictionary. In follow-ups, "content_security_policy.sandbox" and "content_security_policy.content_scripts" will be implemented.

BUG=914224

Change-Id: I1e3a3b1757d1417bf12c3f4fb2fa652fe8bc29e8
Reviewed-on: https://chromium-review.googlesource.com/c/1373253
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616847}
13 files changed