Add threaded backend sink to DM. Fix a scheduler bug.

The bug is spotted by the new sink. I'd expect more bugs to be revealed
and fixed using the new sink.

Bug: skia:
Change-Id: I6b0c9267079fbd6149004f0ecd55ddb179702588
Reviewed-on: https://skia-review.googlesource.com/109720
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
6 files changed