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This is barely functional, but provides a framework which can be fleshed
out. The important point to review is that functionality is unchanged
when the feature flag is not set.

Bug: 941120
Change-Id: I9c0d19458bb8d5e1872144722232041b9a398304
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531530Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644827}