GitLab

* Add sequence checks.

* Replace base::PostTask*() calls with calling PostTask*() methods
  directly on TaskRunners.

* Use PostTaskAndReplyWithResult() instead of equivalent implementation
  based on PostTaskAndReply().

Change-Id: I291e6d9dea744bd1eb7c020f86cacf0f1ab40ae1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633728
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844905}