select the associated device if it is the default output device

When there are multiple associated devices, there is no good way to determine which is the active device we should use. We use the device iff the associated device is the default output device, otherwise we should return an empty string to indicate we can't determine the correct associated device, then the users will use the default output device instead.

BUG=315093
TEST=http://webrtc.googlecode.com/svn/trunk/samples/js/demos/html/local-audio-rendering.html verify the audio works with the built-in device in a mac station machine.
R=tommi@chromium.org

Review URL: https://codereview.chromium.org/59043004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233256 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233256