Use default value for incoming policy when required entry not found in incoming policy

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}
3 files changed