Reduce flakiness of voe_auto_test MixingTest by checking dumped audio size

BUG=webrtc:2925

TEST=passed_all_trybots
R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18479005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6193 4adac7df-926f-26a2-2b94-8c16560cd09d