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

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
 
 #include "SkTypes.h"
 #include "SkThread_platform.h"
 
 /****** SkThread_platform needs to define the following...
 
 int32_t sk_atomic_inc(int32_t*);
 int32_t sk_atomic_add(int32_t*, int32_t);
 int32_t sk_atomic_dec(int32_t*);
 int32_t sk_atomic_conditional_inc(int32_t*);
+int32_t sk_atomic_unprotected_read(const volatile int32_t &);
 
 class SkMutex {
 public:
     SkMutex();
     ~SkMutex();
 
     void    acquire();
     void    release();
 };
 
@@ skipping 27 matching lines @@
             fMutex->release();
             fMutex = NULL;
         }
     }
 
 private:
     SkBaseMutex* fMutex;
 };
 
 #endif