remove SkThread, using std::thread instead

Change-Id: I871dd5eea4496e87c206b46d9eae81cb521b11ce
Reviewed-on: https://skia-review.googlesource.com/65103
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
14 files changed