Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

This will avoid the need to store the main thread TaskRunner in
a member variable in tests that need to post to the main thread
from other threads (e.g.
https://chromium.googlesource.com/chromium/src/+/134190a648fe48fed0486cf8889569863e4a544c/media/capture/video/video_capture_device_unittest.cc#109)

BUG=708584
Review-Url: https://codereview.chromium.org/2885783002
Cr-Commit-Position: refs/heads/master@{#472111}
Committed: https://chromium.googlesource.com/chromium/src/+/812f416e1368152c31893028eeee7e85bd25e827

[fdoray, 2017-05-16 12:53:10]

The CQ bit was checked by fdoray@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-16 12:53:38]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[fdoray, 2017-05-16 12:54:08]

fdoray@chromium.org changed reviewers:
+ gab@chromium.org, robliao@chromium.org

[fdoray, 2017-05-16 12:54:09]

PTAL

[commit-bot: I haz the power, 2017-05-16 14:25:59]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-16 14:25:59]

Dry run: This issue passed the CQ dry run.

[gab, 2017-05-16 15:24:03]

The CQ bit was checked by gab@chromium.org

[gab, 2017-05-16 15:24:03]

lgtm

[commit-bot: I haz the power, 2017-05-16 15:24:28]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-05-16 15:34:55]

CQ is committing da patch.

Bot data: {"patchset_id": 20001, "attempt_start_ts": 1494948243101720,
"parent_rev": "1deb45d63a2d3df56f86f63bd5672219d8150ca9", "commit_rev":
"812f416e1368152c31893028eeee7e85bd25e827"}

[commit-bot: I haz the power, 2017-05-16 15:35:08]

Description was changed from

==========
Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

This will avoid the need to store the main thread TaskRunner in
a member variable in tests that need to post to the main thread
from other threads (e.g.
https://chromium.googlesource.com/chromium/src/+/134190a648fe48fed0486cf88895...)

BUG=708584
==========

to

==========
Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

This will avoid the need to store the main thread TaskRunner in
a member variable in tests that need to post to the main thread
from other threads (e.g.
https://chromium.googlesource.com/chromium/src/+/134190a648fe48fed0486cf88895...)

BUG=708584

Review-Url: https://codereview.chromium.org/2885783002
Cr-Commit-Position: refs/heads/master@{#472111}
Committed:
https://chromium.googlesource.com/chromium/src/+/812f416e1368152c31893028eeee...
==========

[commit-bot: I haz the power, 2017-05-16 15:35:09]

Committed patchset #2 (id:20001) as
https://chromium.googlesource.com/chromium/src/+/812f416e1368152c31893028eeee...