Make ReconfigureVideoEncoder use current bitrate.

Prevents bitrate drops when changing resolution etc.

R=stefan@webrtc.org
BUG=1667

Review URL: https://webrtc-codereview.appspot.com/24069004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7493 4adac7df-926f-26a2-2b94-8c16560cd09d