[scheduling] Use ScopedTaskEnvironment instead of MessageLoop in web_request

MessageLoop will go away, eventually.

BUG=891670

Change-Id: Iee4eb7b25fd741d9344b280bc2b5f2748d18f83f
Reviewed-on: https://chromium-review.googlesource.com/c/1417453
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#623678}
1 file changed