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This correspond to step 1.b from the "Unify CSP" document:
https://docs.google.com/document/d/1v5mJnXJ5dSVXE_rgvJnNM9bzH0ni0YzdhPQ7GLqyhao

This merges:
 - content::CSPSourceList
 - network::mojom::CSPSourceList

Only the mojo one remains.

Future follow-up will apply do the same CSPDirective and ContentSecurityPolicy.

The goal at the end is to have an unique mojo data structure for representing
Content-Security-Policy.

Bug: 1021462
Change-Id: Ie9f826dec8f6e0f3ec9e8fd38bc606250bd6fc6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022948Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736808}