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This patch is straightforward, but it is hard to write test.
We land this patch first, and check the result in dashboard.

This feature is behind flag, disabled by default.

Bug: 839646
Change-Id: Id81843640f28416da440eb3d44f0933121bc67a3
Reviewed-on: https://chromium-review.googlesource.com/c/1306514
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Paul Miller <paulmiller@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605088}