SkSemaphore: use SYNC_POLICY_LIFO on Mach.

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:

Committed: https://skia.googlesource.com/skia/+/2a4685f8bb1bf3cf098c5fee78a71e2f4204e428

[mtklein_C, 2015-06-17 18:14:32]

mtklein@chromium.org changed reviewers:
+ bungeman@google.com

[mtklein_C, 2015-06-17 18:14:36]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-17 18:14:56]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192433006/1

[commit-bot: I haz the power, 2015-06-17 18:20:11]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-17 18:20:12]

Dry run: This issue passed the CQ dry run.

[bungeman-skia, 2015-06-17 18:48:12]

The CQ bit was checked by bungeman@google.com

[bungeman-skia, 2015-06-17 18:48:13]

lgtm

[commit-bot: I haz the power, 2015-06-17 18:48:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1192433006/1

[commit-bot: I haz the power, 2015-06-17 18:49:08]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/2a4685f8bb1bf3cf098c5fee78a71e2f4204e428