SkSemaphore: use SYNC_POLICY_LIFO on Mach.

src/core/SkSemaphore.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSemaphore.h"
 
 #if defined(SK_BUILD_FOR_MAC) || defined(SK_BUILD_FOR_IOS)
     #include <mach/mach.h>
     struct SkSemaphore::OSSemaphore {
         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); }
 
         void signal(int n) { while (n --> 0) { semaphore_signal(fSemaphore); } }
         void wait() { semaphore_wait(fSemaphore); }
     };
 #elif defined(SK_BUILD_FOR_WIN32)
     struct SkSemaphore::OSSemaphore {
         HANDLE fSemaphore;
 
@@ skipping 50 matching lines @@
     }
 }
 
 void SkSemaphore::wait() {
     // We only wait() on the OS semaphore if the count drops <= 0,
     // i.e. when we need to make this thread sleep to wait for it to go back up.
     if (fCount.fetch_add(-1, sk_memory_order_acquire) <= 0) {
         fOSSemaphore->wait();
     }
 }