Flush before snap SkThreadedBMPDevice

Bug: skia:
Change-Id: I3551634cc3d066777b160adafa861f5c8f57d4e8
Reviewed-on: https://skia-review.googlesource.com/118961
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
2 files changed