[scheduler] Simplifying TaskRunners for code health.

The various TaskRunner interfaces (TaskRunner, SequencedTaskRunner and
SingleThreadTaskRunner) had a lot of the same code, and the differences in
handling pre-native tasks were obfuscated. This is an attempt to simplify
them, which will facilitate further work on adding postDelayedTask.

Bug: 863341
Change-Id: I43c28287bddb8c91d4f67ca2887cb32b613b6bdb
Reviewed-on: https://chromium-review.googlesource.com/c/1386546
Commit-Queue: Karolina Soltys <ksolt@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620779}
6 files changed