Add threaded backend to viewer

Bug: skia:
Change-Id: Ibf672921f8a05705e7262aad0b1f3f1e6fc0ef9c
Reviewed-on: https://skia-review.googlesource.com/75382
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
2 files changed