GitLab

Add an ability to subscribe to the notifications about
ScopedTaskEnviroments being created and destroyed, which should help
with content unittests, where we have multiple ScopedTaskEnvironments
changing under one SequenceManager.

R=gab@chromium.org
CC=alexclarke@chromium.org
BUG=892139

Change-Id: Ib56fd0f1b465edd3a6dd6693b36ef3d8b9234140
Reviewed-on: https://chromium-review.googlesource.com/c/1251449
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600097}