Delay PeerConnectionWrapper destruction until data channels are closed

Due to a recent WebRTC change, if the PeerConnection is closed before
the data channels complete their close dance then an error is raised on
the other side of the connection.  For our purposes, we can't tell the
difference between a connection which was closed on the host-side and a
connection that was dropped due to network conditions, host crash, etc.

This CL adds some logic to delay destruction of the PeerConnection until
after the main data channels (event and control) report that they have
closed.  These channels are wrapped in several layers of abstraction so
instead, mostly to make it simple for the host to handle both WebRTC and
Chromotocol connections.  The simplest way I could find to track the
channel state was to store refptrs for the event and control channels in
the WebrtcTransport class and then query them (requires polling) after
so often until they report they have successfully closed.  If for some
reason this takes way longer than usual, we have a max timeout of 5
seconds which will just close the peer connection.

Bug: 1083269
Change-Id: I5a42282d7dd4f74ad734e2fc1a955e42c67e4ce7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2197415
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769459}