SkSemaphore: use SYNC_POLICY_LIFO on Mach.

Turns out _LIFO does work as we expected, keeping N < M threads active when we
have M live threads but only N units of work to do at a time.  Also as we
expected _FIFO does round-robin through the threads.

Performance doesn't seem to be affected, but let's do it anyway.
If nothing else this makes profiles a little easier to read.

I don't see POSIX or Windows equivalents.

BUG=skia:

Review URL: https://codereview.chromium.org/1192433006
1 file changed