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Since the UI thread is the audio thread on OSX, we should be careful
about what we execute during startup.  Device enumeration and creation
of the device listener can be expensive, so don't do them until the
first stream is created.

BUG=288759
TEST=none

Review URL: https://codereview.chromium.org/154543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251443 0039d316-1c4b-4281-b951-d872f2087c98