Fix deadlock in Android stopCapture() call.

BUG=3467
R=braveyao@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/18789004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6673 4adac7df-926f-26a2-2b94-8c16560cd09d