Enable unified garbage collections part two

This flag overrides the default Blink value for content layer.

Sheriffs: This is a dry-run of the feature and will be reverted after
cycling through the waterfall and performance bots.

Tbr: jochen@chromium.org, lfg@chromium.org
Bug: 843903
Change-Id: Ie362301e4874198eb775346d051bf8267837aaf8
Reviewed-on: https://chromium-review.googlesource.com/c/1320970
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606446}
7 files changed