AudioMirroringManager becomes a global LazyInstance.

AudioMirroringManager becomes a global LazyInstance.

While attempting to resolve a flaky browser_test, it became clear that
AudioMirroringManager was not outliving its use.  In the original change
that introduced this class, it was instantiated and owned by
BrowserMainLoop as a matter of convenience.  However, with some
debugging, it's clear that it must outlive objects that can outlive
BrowserMainLoop (e.g., WebContentsAudioInputStream).

Side notes: I've checked the feasibility of other solutions, confirming
that the shutdown of AudioManager does NOT guarantee complete teardown
of an AudioInputStream, so it's not sufficient to simply change the
destruction order of the objects in BrowserMainLoop to resolve this
problem.  As AudioMirroringManager provides a browser-wide service and
owns no objects, it seems reasonable for it to exist as a global
LazyInstance.

BUG=396413
Committed: https://crrev.com/8b4f7fa775288ec5306b913f133830ed5f771064
Cr-Commit-Position: refs/heads/master@{#292258}

[miu, 2014-08-27 19:54:09]

miu@chromium.org changed reviewers:
+ dalecurtis@chromium.org, sky@chromium.org

[miu, 2014-08-27 19:54:58]

dalecurtis: PTAL.

sky: Need OWNERS stamp for deletions in browser_main_loop.* and
render_process_host_impl.cc.

https://codereview.chromium.org/499483003/diff/1/content/browser/media/captur...
File content/browser/media/capture/audio_mirroring_manager.cc (left):

https://codereview.chromium.org/499483003/diff/1/content/browser/media/captur...
content/browser/media/capture/audio_mirroring_manager.cc:16: class
ReentrancyGuard {
I removed this since: 1) It's never triggered, and 2) I don't see how any of the
calling code paths could ever attempt reentrancy.  IIRC, it was a concern on an
early iteration of this design long ago, but is certainly now irrelevant.

[DaleCurtis, 2014-08-27 20:25:50]

lgtm

[sky, 2014-08-27 21:40:34]

LGTM

[miu, 2014-08-27 21:54:11]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-08-27 21:55:19]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/miu@chromium.org/499483003/1

[commit-bot: I haz the power, 2014-08-27 23:04:06]

Committed patchset #1 (id:1) as 033c98e7d33ea0e93b797b9b3e69e9a12ca66a50

[commit-bot: I haz the power, 2014-09-10 02:55:18]

Patchset 1 (id:??) landed as
https://crrev.com/8b4f7fa775288ec5306b913f133830ed5f771064
Cr-Commit-Position: refs/heads/master@{#292258}