Reduce mixing threshold in test to avoid flakiness.

Flake observed here:
http://chromegw/i/client.webrtc/builders/Win32%20Release%20%5Blarge%20tests%5D/builds/953/steps/voe_auto_test/logs/stdio

TBR=andresp

Review URL: https://webrtc-codereview.appspot.com/8489004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@5530 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed