Fix bugs with fence sync API on iOS devices

- Actually request extension version of fence sync functions
- Fix incorrect usage of dlopen/dlsym
- Also fixed same bugs in Mac code, although we never hit that
  code path.

Should fix iOS devices, giving more accurate (and less spammy)
results from nanobench.

Bug: skia:
Change-Id: I3456b301ef9b0b6559160d1d21c77bd93139d39a
Reviewed-on: https://skia-review.googlesource.com/57740
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed