port most SkAtomic<T> to std::atomic<T>

The only SkAtomic<T> left are the ones overriding the default memory
order in SkPath.  I think I'd like to try switching them to std::atomic
too, but in another CL.

Trim unused APIs in SkAtomics.h.

Change-Id: Ia1c283355902ccb8fcdad70cdf27bb577e8ca407
Reviewed-on: https://skia-review.googlesource.com/146529
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
13 files changed