Added url params arbr, asbr, vrbr & vsbr.

Added url params arbr, asbr, vrbr & vsbr to apprtc demo to control audio
and video send/receive bitrate.

BUG=3166
Modified apprtc code to handle url params arbr, asbr, vrbr & vsbr.

Follow up to the changes suggested by Justin/Ami.
More changes to apprtc code for handling  url params asbr, arbr, vsbr and vrbr.

Updated code, based on suggestions from Justin.
Continuation to apprtc url params for adjusting bitrate.

Added nextMLineIndex as discussed with Justin to find the cline index
and bandwidth lines.
Final commit for adding url params to set audio/video bitrate in apprtc.

Minor change in for loop.
Merge pull request #7 from GoogleChrome/vikas_webrtc

Added url params arbr, asbr, vrbr & vsbr.