[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in subresource_filter

MessageLoop will go away, eventually.

BUG=891670

Change-Id: I3fcddf556b24d65f93a7d575093d89ab45e5e22e
Reviewed-on: https://chromium-review.googlesource.com/c/1352783
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#614369}
1 file changed