Marina, could you land https://chromium-review.googlesource.com/c/chromium/src/+/1119914/10 and add a missing test there?
This fix is needed for ChromeOS work.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9acaaa4312f3c35a59a42f0f29742098f5ad3508

commit 9acaaa4312f3c35a59a42f0f29742098f5ad3508
Author: Olga Sharonova <olka@chromium.org>
Date: Wed Jul 25 10:19:36 2018

Allow blocking when audio::OutputDevice joins the audio thread.


BUG= 858987 

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ic61830a546ffe6873fef4db18a49d03031293fe7
Reviewed-on: https://chromium-review.googlesource.com/1119914
Reviewed-by: Marina Ciocea <marinaciocea@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Max Morin <maxmorin@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577832}
[modify] https://crrev.com/9acaaa4312f3c35a59a42f0f29742098f5ad3508/base/threading/thread_restrictions.h
[modify] https://crrev.com/9acaaa4312f3c35a59a42f0f29742098f5ad3508/media/audio/audio_device_thread.h
[modify] https://crrev.com/9acaaa4312f3c35a59a42f0f29742098f5ad3508/services/audio/public/cpp/output_device.cc
[modify] https://crrev.com/9acaaa4312f3c35a59a42f0f29742098f5ad3508/services/audio/public/cpp/output_device.h