GitLab

There may be remaining messages in the deferred queue on unclean teardown.
Delete those.

Also, a little bit of cleanup in the area:
- GpuChannel doesn't need to be refcounted
- It's ok to copy WeakPtr across threads, as long as they're only dereferenced
on the thread that the factory is destroyed. So no need for the extra
indirection.

BUG=374843

Review URL: https://codereview.chromium.org/299003004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@272584 0039d316-1c4b-4281-b951-d872f2087c98