[base] Move SkSpinlock and SkSharedMutex to src/base

No usages in include/ nor third-party clients, so this should be a
trivial move.

Change-Id: I6b481872b9e449c8197c95ae4107d4ef94e17994
Bug: skia:13983
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/694696
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Herb Derby <herb@google.com>
26 files changed