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This is behind a flag.
And I added some JavaScript tests.

Design Doc:
https://docs.google.com/document/d/1TovyqqeC3HoO0A4UUBKiCyhZlQSl7jM_F7KbWjK2Gcs/edit#heading=h.7nki9mck5t64

Bug: 999091
Change-Id: I4c34d9b80fe50242c6547662040439a541b652b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1810447Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Canon Mukai <canonmukai@google.com>
Cr-Commit-Position: refs/heads/master@{#701041}