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This gives extension authors a way to test whether their extension
is ready to be removed from the allowlist.

Bug: 846346
Change-Id: Ide112dab32076ac2a554115b7ca815a23385050c
Reviewed-on: https://chromium-review.googlesource.com/c/1368783
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619559}