WebRTC: Support new SDP lines for WebRTC bitrate control

[furkanKotic]

Description

When the following SDP lines are added, SRS rejects the connection. SRS must comply with these values:

a=fmtp:x-google-max-bitrate=2000000
a=fmtp:x-google-max-bitrate=1500000
a=fmtp:x-google-start-bitrate=1800000

1. SRS Version: v5.0-b4

[winlinvip]

You are right, SRS should improve ability on WebRTC functionalbilities and SDP features. Currently SRS only support very basic WebRTC streaming.

Patch welcome.