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Currently, DocumentLoader sets FrameLoader::frame_owner_sandbox_flags_
only to immediately read that state back. Give FrameLoader a
GetForcedSandboxFlags() helper to access its permanent flags, then
have DocumentLoader assemble the state it needs rather than using
FrameLoader::EffectiveSandboxFlags().

The only other use of FrameLoader::EffectiveSandboxFlags() is a
test-only case. Switch it to use PendingEffectiveSandboxFlags(), since
all the tests seem to be happy with it.

Change-Id: Id58eda10c18843b3e2ddc4b962513507e2e66aeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333335
Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794799}