Have MessageQueue return whether a task was posted after queue killed

Messages posted to queues after the queue is killed aren't run. During
worker termination, this can lead to callers believing a message
was posted and will be handled, but the message is instead dropped
on the floor. Return the queue state when posting, and bubble this
up to the public API.

BUG=226256

Review URL: https://chromiumcodereview.appspot.com/19267026

git-svn-id: svn://svn.chromium.org/blink/trunk@154499 bbb929c8-8fbe-4397-9dbb-9b2b20218538