switch program cache to try-acquire

No need to contend if someone else is using the cache.

I switched the attributes back to the front of the declaration;
the line for try_acquire_program_cache() was getting long and
I couldn't find any satisfyingly clear place to break it.

Change-Id: I28c24c342b10c043091c0c46bd27153e44308967
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/233058
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed