Let each threaded device in the ThreadSink own its SkExecutor

See https://bugs.chromium.org/p/skia/issues/detail?id=7838#c17

Bug: skia:7838
Change-Id: I98e723e7a2d5f7506d019e28797c34e12582f47b
Reviewed-on: https://skia-review.googlesource.com/125283
Commit-Queue: Yuqian Li <liyuqian@google.com>
Auto-Submit: Yuqian Li <liyuqian@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed