Completed the functionalities of SrtpTransport.

The SrtpTransport takes the SRTP responsibilities from the BaseChannel
and SrtpFilter. SrtpTransport is now responsible for setting the crypto
keys, protecting and unprotecting the packets. SrtpTransport doesn't know
if the keys are from SDES or DTLS handshake.

BaseChannel is now only responsible setting the offer/answer for SDES
or extracting the key from DtlsTransport and configuring the

SrtpFilter is used by BaseChannel as a helper for SDES negotiation.


Cr-Original-Commit-Position: refs/heads/master@{#19636}
Cr-Mirrored-Commit: e683c6871fef24d3ff64f085d6bc0e965f17fcf7
13 files changed