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I intentionally didn't add "Foreground" to non "Background" workers as
they may be used for background tasks in some configurations and I don't
want it to be confusing for developers.

R=fdoray@chromium.org

Bug: 839525
Change-Id: I8c1e928914aa6d2e35fedc1db4e8a639d39116b2
Reviewed-on: https://chromium-review.googlesource.com/1044501
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556169}