mus-gpu: Make sure the channel request callback is always called.

The gpu host (i.e. window server) can attempt to shut down while there
are in-flight EstablishChannel requests to the gpu service from various
clients. It is necessary to run these callbacks when shutting down, to
avoid crashes about mojom-callbacks that never ran.

BUG=611505

Change-Id: I45b790dc20ebc4d4a138aa01ed043d67ed9e6569
Reviewed-on: https://chromium-review.googlesource.com/738535
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511805}
3 files changed