[scheduler] A migration from SERIAL_EXECUTOR to SequencedTaskRunner.

Bug: 863341
Change-Id: I47a27a735c473639b98da80967bbcb60c600cf5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702414
Reviewed-by: Karolina Soltys <ksolt@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Karolina Soltys <ksolt@chromium.org>
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678671}
1 file changed