Base: Allow PostTaskAndReply w/o sequence when target TaskRunner doesn't accept tasks.

This is to facilitate writing code that calls PostTaskAndReply
during late shutdown.

Bug: 922938
Change-Id: I481990274593da527d8b567a014cc5ae8b79b345
Reviewed-on: https://chromium-review.googlesource.com/c/1442873
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628765}
2 files changed