Prevent AudioDeviceThread from starting if clients have called Stop().

      Due to ordering of messages between the media thread and the IO thread it was possible for AudioOutputDevice to receive OnStreamCreated() after AudioRendererImpl had called Stop(). Since AudioRendererImpl is deleted shortly after calling Stop(), AudioDeviceThread would get started with a callback pointer to a potentially deleted AudioRendererImpl.

BUG=147499
TEST=media_unittests, asan build w/ test file included in bug report

Review URL: https://codereview.chromium.org/10938006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@157378 0039d316-1c4b-4281-b951-d872f2087c98