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Currently the "content_security_policy" manifest key is restricted to trunk.
However it is mandatory for Mv3 extensions. Hence, always allow it if the
manifest version is 3.

Also, make some other minor cosmetic changes.

BUG=993530

Change-Id: Ifca0c588d923a36644002e865f853d74533d7356
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815277
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698986}