Add support for the live source for WebRTC

Add support for the live source for WebRTC

Media streamer provides several APIs to change its states
such as prepare(), play() and pause().
During prepare(), it is required for WebRTC node to exchange
negotiation messages via signaling server controlled by
application. But the implimentation of webrtcbin invokes the
'on-negotiation-needed' callback as soon as the first source
buffer arrives. This patch enables source plugins which do not
have the preroll buffer to work well with WebRTC node.

[Version] 0.1.95
[Issue Type] Improvement

Change-Id: I905351c0fd1a422d8e0c64d79995408c5720ea2e
Signed-off-by: Sangchul Lee <sc11.lee@samsung.com>