Set number of temporal layers for VideoSendStream.

Introduces a mapping between EncoderConfig and VideoCodec. More
specifically it also removes an assert that there should be no set
temporal layers in the new API, which is wrong and was temporary.

R=stefan@webrtc.org
BUG=1788

Review URL: https://webrtc-codereview.appspot.com/25619004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7256 4adac7df-926f-26a2-2b94-8c16560cd09d