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This CL moves handling of the "sandbox.content_security_policy" to the
CSPHandler from SandboxedPageHandler. This should have no behavior change. This
is in preparation of introducing the "content_security_policy.sandbox" manifest
key.

BUG=914224

Change-Id: I27e839a09211d7fccc0104e6195d6c123cca8ef2
Reviewed-on: https://chromium-review.googlesource.com/c/1391877
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619817}