Add more thread annotations to SkStrike

* remove internalGetHead/Tail
* make validate private because fLock must be held.

Change-Id: If998cfe8ba0414e4520807034f841e40b94aa206
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260284
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed