Add pooling support on iOS.

This replaces the `thread_local` attribute with `pthread_setspecific`
and `pthread_getspecific`. I don't have easy access to iOS 8/9 for
testing purposes, but on Mac OS X, this implementation works and
benchmarks the same as the `thread_local` implementation.

Change-Id: I86db88c24d59d946adb66141b32733ebf5261c76
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328837
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
1 file changed