Backport some tests in /printing to SingleThreadTaskEnvironment

These tests were previously migrated from single-threaded MessageLoop to
a multi-threaded TaskEnvironment (then named ScopedTaskEnvironment) as
part of crbug.com/891670.

//base OWNERS decided in retrospect that it was better to keep a
single-threaded option for TaskEnvironment and introduced
SingleThreadTaskEnvironment. This CL retrofits that decision for
/printing.

This CL is a no-op if it passes CQ.

This CL was uploaded by git cl split.

[email protected]

Bug: 891670
Change-Id: I6077f3d2fb31e4fa16a64be3cf209e25ef2998db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787179
Auto-Submit: Gabriel Charette <[email protected]>
Reviewed-by: Lei Zhang <[email protected]>
Commit-Queue: Lei Zhang <[email protected]>
Cr-Commit-Position: refs/heads/master@{#693856}
1 file changed