[base] Make PostTask destination explicit in /chrome/browser/chromeos/app_mode

Task APIs v3 will all have an explicit destination. In the past
"default traits" meant ThreadPool. These CLs are thus a no-op
except for the few callers that clearly wanted to post a QuitClosure()
on the current thread (posting a QuitClosure() to a parallel task will
merely bounce it back to the current thread anyways). Those few callers
were migrated to ThreadTaskRunnerHandle::Get().

These changes are thus required to phase out task APIs v2 (post_task.h)
Design doc: https://docs.google.com/document/d/1tssusPykvx3g0gvbvU4HxGyn3MjJlIylnsH13-Tv6s4/edit

This specific change is a subset of
https://chromium-review.googlesource.com/c/chromium/src/+/2216733

This CL was uploaded by git cl split.

R=anqing@chromium.org

Bug: 1026641
Change-Id: I89a068370f65f68feb17379b74031c7f8011f289
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216734
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Anqing Zhao <anqing@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772385}