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As part of on-going cleanup to dispatch an event on corruption
detection, add a "Force schema downgrade" option on an internal
page so that web developers can trigger corruption in a way
that will allow them to test and verify their apps correctly
handle the event.

Bug: 876787
Change-Id: If03d47ce8ec3bc930e9a5a8a7e04835116cf4a36
Reviewed-on: https://chromium-review.googlesource.com/1183586
Commit-Queue: Chase Phillips <cmp@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586434}