Reland "Make Chrome_DevToolsADBThread shutdown gracefully in tests"

Relanding https://chromium-review.googlesource.com/c/chromium/src/+/1238468

The fix can be seen in the delta between patch set 1 and 2.

Original patch description:
Make Chrome_DevToolsADBThread shutdown gracefully in tests

To land the BrowserUIThreadScheduler we need in tests for all threads
to stop using base::PostTaskWithTraits before the scheduler is
deleted. Currently this is true for the UI and IO threads but it's
not true for Chrome_DevToolsADBThread.

This patch makes sure references retaining
AndroidDeviceManager::HandlerThread are released in tests
which triggers thread shutdown, fixing the problem.

TBR=dgozman@chromium.org,finnur@chromium.org,mlamouri@chromium.org

Bug: 863341, 872372, 888994
Change-Id: Ie9298d652cad2df6ad93d19cfb3dd5b76fdcf339
Reviewed-on: https://chromium-review.googlesource.com/1243115
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593970}
8 files changed