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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

Review URL: https://codereview.chromium.org/499483003

Cr-Commit-Position: refs/heads/master@{#292258}