GitLab

Add early returns when returning to the main thread, to ensure the
ExecutionContext or LocalFrame is always valid before use.
Also, add some comments for consistency between subclasses.

Change-Id: I6abe00c90405c44214a45c8b5d02e070db767956
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551304
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836960}