replace SkSharedMutex

I am debugging an issue with SkSharedMutex and noticed
how sparsely it is used.  That got me curious to see if
we can replace it with a std::shared_mutex (from C++17).

Bug: skia:10177
Change-Id: I1ce4d2a5897af198d6ae5fb850548ff917a58f50
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285691
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
7 files changed