Also fixup some implementations that were lagging behind per the lack of
pure virtual not having enforced everything yet.

Also fixed recently introduced
PredictablePlatform::CallDelayedOnWorkerThread() to ignore delayed tasks
after realizing the intent is to intercept worker tasks instead of
sending them to |platform_|.

Node.js migrated off these APIs @
https://github.com/v8/node/pull/69

R=ahaas@chromium.org, yangguo@chromium.org

Bug: chromium:817421
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I92171f213b5fc64ab1f21e8eec72738f5ce228bd
Reviewed-on: https://chromium-review.googlesource.com/1045310
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53223}