Fix and enable CanReceiveFec test.

Test relied on the first protected media packet that was dropped to
actually be rendered, while rendering it could have been skipped on slow
systems due to newer frames being decoded before rendering happens.

R=stefan@webrtc.org
BUG=3269

Review URL: https://webrtc-codereview.appspot.com/25159004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7696 4adac7df-926f-26a2-2b94-8c16560cd09d