GitLab

This changes the behavior for opting output devices into ducking so that ducking
is only enabled if an input device has already been opened with ducking enabled.

Entering the ducking session is now done for local streams as well, including
HTMLMediaElement audio rendering.  This allows the communication application to
also play back unducked ambient sounds.

On the browser side, when we open the default communication device(s) (input and
output), we now initialize them into a session that is separate from other
audio.  This makes it much easier to observe, control and troubleshoot what is
happening during a comm session.  It also fixes a problem whereby opening up an
output communications device before opening up an input communications device
would cause the output device to get ducked for some reason (and not opted out
of ducking).

Review URL: https://codereview.chromium.org/367923004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281814 0039d316-1c4b-4281-b951-d872f2087c98