If the callback has been cancelled (e.g due to weak ptr and that object has
been destroyed), run base::File::Close() on the thread that allows I/O.
TEST=tests using CdmFile still pass
Commit-Queue: John Rummell <firstname.lastname@example.org>
Reviewed-by: Joshua Bell <email@example.com>
Reviewed-by: Xiaohan Wang <firstname.lastname@example.org>