Delete Channels without ChannelManager lock.
Delete Channels without ChannelManager lock.

Triggered Helgrind error, as deleting a Channel will also unregister a
module which has called GetChannel(), resulting in a cyclic lock graph.
This change will also allow other threads to access the ChannelManager
instance while Channels are deleted.

R=xians@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1946005

git-svn-id: 917f5d3ca488f358c4d40eaec14422cf392ccec9@4505 4adac7df-926f-26a2-2b94-8c16560cd09d

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,mwgoldsmith/libilbc,mwgoldsmith/libilbc,mwgoldsmith/ilbc,TimothyGu/libilbc,mwgoldsmith/libilbc,mwgoldsmith/ilbc,ShiftMediaProject/libilbc,mwgoldsmith/ilbc,mwgoldsmith/ilbc,mwgoldsmith/libilbc,TimothyGu/libilbc,TimothyGu/libilbc