Extensions: Implement "content_security_policy.isolated_world" manifest key.

This CL implements the "content_security_policy.isolated_world" manifest key. In
a later CL, the parsed CSP will be plumbed to the renderer.

BUG=914224

Change-Id: Ia13904d760c40b20a3309c521d44666c2dfbc0c7
Reviewed-on: https://chromium-review.googlesource.com/c/1406233
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625493}
18 files changed