Add DCHECK when shutdown behavior is used to post to UI/IO threads on iOS.

The BLOCK_SHUTDOWN behavior is currently not respected for tasks posted
to the UI/IO threads. Indeed, nothing prevents the UI/IO run loop to
exit before all BLOCK_SHUTDOWN tasks have been processed.

Bug: 872372, 908348
Change-Id: If14cfa5af4ddef5a075f3113c74ab03ad30cd5d7
Reviewed-on: https://chromium-review.googlesource.com/c/1352347
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616278}
2 files changed