Add SkBarriers to ports.

include/core/SkThread.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkThread_DEFINED
 #define SkThread_DEFINED
 
@@ skipping 42 matching lines @@
     int32_t prev;
     do {
         prev = *addr;
         if (0 == prev) {
             break;
         }
     } while (!sk_atomic_cas(addr, prev, prev+1));
     return prev;
 }
 
+// SK_BARRIERS_PLATFORM_H must provide implementations for the following declarations:
+
+/** Prevent the compiler from reordering across this barrier. */
+static void sk_compiler_barrier();
+
+/** Read T*, with at least an acquire barrier.
+ *
+ *  Only needs to be implemented for T which can be atomically read.
+ */
+template <typename T> T sk_acquire_load(T*);
+
+/** Write T*, with at least a release barrier.
+ *
+ *  Only needs to be implemented for T which can be atomically written.
+ */
+template <typename T> void sk_release_store(T*, T);
+
+#include SK_BARRIERS_PLATFORM_H
+
 /** SK_MUTEX_PLATFORM_H must provide the following (or equivalent) declarations.
 
 class SkBaseMutex {
 public:
     void acquire();
     void release();
 };
 
 class SkMutex : SkBaseMutex {
 public:
@@ skipping 35 matching lines @@
             fMutex = NULL;
         }
     }
 
 private:
     SkBaseMutex* fMutex;
 };
 #define SkAutoMutexAcquire(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexAcquire)
 
 #endif