Data race fix: Replace use of MessageLoop raw pointer in MSAudioProcessor.

Data race fix: Replace use of MessageLoop raw pointer in MSAudioProcessor.

This change replaces the use of a raw pointer to the main thread's
MessageLoop, by content::MediaStreamAudioProcessor, with a
scoped_refptr to the main thread's task runner. This fixes a
TSan-reported shutdown data race which was caused by the MessageLoop
releasing its task runner at the same time the audio thread was using
the MessageLoop::task_runner() accessor method.

BUG=647178
Review-Url: https://chromiumcodereview.appspot.com/2425353002
Cr-Commit-Position: refs/heads/master@{#426318}
(cherry picked from commit a4717b9caba5b1fb1e4455af8932a3a870a9aecf)

Committed: https://chromium.googlesource.com/chromium/src/+/6f8fc5767534da6ea3682ab3bde09e782909467b

[miu, 2016-10-24 21:47:11]

Description was changed from

==========
Data race fix: Replace use of MessageLoop raw pointer in MSAudioProcessor.

This change replaces the use of a raw pointer to the main thread's
MessageLoop, by content::MediaStreamAudioProcessor, with a
scoped_refptr to the main thread's task runner. This fixes a
TSan-reported shutdown data race which was caused by the MessageLoop
releasing its task runner at the same time the audio thread was using
the MessageLoop::task_runner() accessor method.

BUG=647178

Review-Url: https://chromiumcodereview.appspot.com/2425353002
Cr-Commit-Position: refs/heads/master@{#426318}
(cherry picked from commit a4717b9caba5b1fb1e4455af8932a3a870a9aecf)
==========

to

==========
Data race fix: Replace use of MessageLoop raw pointer in MSAudioProcessor.

This change replaces the use of a raw pointer to the main thread's
MessageLoop, by content::MediaStreamAudioProcessor, with a
scoped_refptr to the main thread's task runner. This fixes a
TSan-reported shutdown data race which was caused by the MessageLoop
releasing its task runner at the same time the audio thread was using
the MessageLoop::task_runner() accessor method.

BUG=647178

Review-Url: https://chromiumcodereview.appspot.com/2425353002
Cr-Commit-Position: refs/heads/master@{#426318}
(cherry picked from commit a4717b9caba5b1fb1e4455af8932a3a870a9aecf)

Committed:
https://chromium.googlesource.com/chromium/src/+/6f8fc5767534da6ea3682ab3bde0...
==========

[miu, 2016-10-24 21:47:13]

Committed patchset #1 (id:1) manually as
6f8fc5767534da6ea3682ab3bde09e782909467b.