[ios] Store TaskRunners instead of threads in WebThreadImpl.

Track thread states explicitly instead of simply checking
whether a TaskRunner pointer exists or not.

BUG=826465

Change-Id: Id0f84e7eedc6b9f6568aec629abbb86f46979efa
Reviewed-on: https://chromium-review.googlesource.com/c/1340827
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616319}
3 files changed