Added a builtin audio decoder factory to the default PeerConnectionFactory constructor.
Added a builtin audio decoder factory to the default PeerConnectionFactory constructor.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2070833002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13186}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc