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The current threshold came from my local experiments and it wouldn't be
necessarily an appropriate value. Make the threshold configurable so
that we can run experiments in the real world.

Bug: 776665
Change-Id: I6ae896439eb50985221258b9109f6f06e3aea178
Reviewed-on: https://chromium-review.googlesource.com/1022410
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552963}