Drain compositor_chan on shutdown, causes deadlocks on constellation

Constellation blocks on SetId by sending a Port through the compositor
channel and waits for a response.  If the compositor is in the process of
shutting down, it will not look in the queue again.  The compositor
requires the constellation to be shut down first, so it sends a message
to shut down and blocks until the constellation finishes, deadlocking.

Only very short lived executions would've been likely to see this deadlock.
auto merge of #1415 : dhedlund/servo/constellation_deadlock, r=pcwalton

Constellation blocks on SetId by sending a Port through the compositor channel and waits for a response.  If the compositor is in the process of shutting down, it will not look in the queue again.  The compositor requires the constellation to be shut down first, so it sends a message to shut down and blocks until the constellation finishes, deadlocking.

Only very short lived executions would've been likely to see this deadlock.

_It is technically still possible to trigger a SetId message between when the compositor finishes running and when the constellation shuts down, but the window for exposure is greatly reduced.  I've only seen it with "-o" so far._