If one of the bundled content is missing in SDP, return false to MaybeEnalbeMuxingSupport().
Verified in chromium. Now the existing content still could work.

BUG=4096
TEST=Manual Test
R=juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/36529004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7926 4adac7df-926f-26a2-2b94-8c16560cd09d