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This CL let document policy use default value when not specified in header.
This CL also lets static content such as about:blank/about:srcdoc to always fulfill required document policy so that they are not blocked.

Change-Id: I14bf7d114fcd838f92ae91f773bde944f65f2c13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2039972
Commit-Queue: Charlie Hu <chenleihu@google.com>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741485}