Have Opus follow Chromium revisions

Before this change, a pinned version of Opus was used in WebRTC. This could lead to WebRTC running a different version of Opus compared to the version used with the corresponding Chromium revision.
This CL pulls in the Opus version the Chromium uses.

BUG=3546
R=kjellander@webrtc.org, niklas.enbom@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15979005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6602 4adac7df-926f-26a2-2b94-8c16560cd09d