Extensions: Introduce "content_security_policy.sandbox" manifest key.

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

BUG=914224

Change-Id: Ia5ed3a2bee901c9babfc02e8f73e9228955bae75
Reviewed-on: https://chromium-review.googlesource.com/c/1391485
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620938}
11 files changed