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Add missing flags to the iframe sandbox supported list, and add tests.
Also remove the runtime config for 'allow-top-navigation-by-user-activation' since the feature has shipped a long time ago.

Bug: 662506
Change-Id: I458210f6133ddd5e30b507214b003ff0fb596761
Reviewed-on: https://chromium-review.googlesource.com/965843
Commit-Queue: Bin Lu <binlu@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543733}