[TaskEnvironment] Mass-migrate away from ScopedTaskEnvironment (1/2)

(well half of them because git cl upload wouldn't let me do
 them all at once...)

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: I44bc376deee9b6c95bafac8d54165174d86a5001
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1756247
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688086}
835 files changed