Cast: Start the sender only if both audio and video are configued

Cast sender creates a new network transport when starting a audio stream
or a video stream. This means during a single session the sending port
is changing. Receiver is confused and cannot send RTCP packets to the
correct port.

Instead of creating a new sender for both audio and video. Start the
sender if both audio and video are configued.

The ideal solution is to have CastSender API to support initialization
of audio and video sender separately. This change is coming later.

R=hubbe@chromium.org
TBR=hubbe@chromium.org
BUG=346822

Review URL: https://codereview.chromium.org/180343003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253622 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253622