[TaskEnvironment] Mass-migrate away from ScopedTaskEnvironment

(second half of
 https://chromium-review.googlesource.com/c/chromium/src/+/1756247)

This is step #1 of the mass migration. Some of these will be
backported to SingleThreadTaskEnvironment in a later phase.

scoped_task_environment.h will also only move in a follow-up CL.

TBR=dcheng@chromium.org

Bug: 992483
Change-Id: Ie80f083eeeb209180ab3c2884f1b566f73549cc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1756248
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#688142}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: dfa3604eca9277f044caaf6ff5744075f26fe81f
1 file changed