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 files changed, 1 insertions, 1 deletions

diff --git a/src/core/SkSemaphore.cpp b/src/core/SkSemaphore.cpp
index 3f19d4fa12..ac62930082 100644
--- a/src/core/SkSemaphore.cpp
+++ b/src/core/SkSemaphore.cpp
@@ -13,7 +13,7 @@
         semaphore_t fSemaphore;
 
         OSSemaphore()  {
-            semaphore_create(mach_task_self(), &fSemaphore, SYNC_POLICY_FIFO, 0/*initial count*/);
+            semaphore_create(mach_task_self(), &fSemaphore, SYNC_POLICY_LIFO, 0/*initial count*/);
         }
         ~OSSemaphore() { semaphore_destroy(mach_task_self(), fSemaphore); }