VideoSendStreamTest.SwapsI420VideoFrames: Initialize frame memory to avoid drmemory errors

R=pbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/27149004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7688 4adac7df-926f-26a2-2b94-8c16560cd09d