Add a 'unique' method to SkRefCnt, document the usage, and add support.

src/ports/SkThread_pthread.cpp

 
 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "SkThread.h"
 #include "SkTLS.h"
 
@@ skipping 92 matching lines @@
     SkAutoMutexAcquire ac(gAtomicMutex);
 
     int32_t value = *addr;
     if (value != 0) ++*addr;
     return value;
 }
 void sk_membar_aquire__after_atomic_conditional_inc() { }
 
 #endif
 
+int32_t sk_atomic_unprotected_read(const volatile int32_t & x) {
+    return x;
+}
+
 #endif // SK_BUILD_FOR_ANDROID
 
 //////////////////////////////////////////////////////////////////////////////
 
 static void print_pthread_error(int status) {
     switch (status) {
     case 0: // success
         break;
     case EINVAL:
         SkDebugf("pthread error [%d] EINVAL\n", status);
@@ skipping 88 matching lines @@
     // and just return NULL if we've never been called with
     // forceCreateTheSlot==true ?
 
     (void)pthread_once(&gSkTLSKey_Once, sk_tls_make_key);
     return pthread_getspecific(gSkTLSKey);
 }
 
 void SkTLS::PlatformSetSpecific(void* ptr) {
     (void)pthread_setspecific(gSkTLSKey, ptr);
 }