Extensions: Restrict mv3 extensions to use "content_security_policy" as dictionary.

This CL fixes an existing TODO and restricts manifest v3 extensions such that
they always use "content_security_policy" manifest key as a dictionary.

BUG=914224

Change-Id: I4b7be7cd9edf64a016d343660e11e4a197c5b409
Reviewed-on: https://chromium-review.googlesource.com/c/1423744
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625515}
5 files changed