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

src/core/SkTRefArray.h

 //
 //  SkTRefArray.h
 //  core
 //
 //  Created by Mike Reed on 7/17/12.
 //  Copyright (c) 2012 __MyCompanyName__. All rights reserved.
 //
 
 #ifndef SkTRefArray_DEFINED
 #define SkTRefArray_DEFINED
@@ skipping 56 matching lines @@
     const T& at(int index) const {
         SkASSERT((unsigned)index < (unsigned)fCount);
         return this->begin()[index];
     }
     const T& operator[](int index) const { return this->at(index); }
 
     // For the writable methods, we assert that we are the only owner if we
     // call these, since other owners are not informed if we change an element.
 
     T* writableBegin() {
-        SkASSERT(1 == this->getRefCnt());
+        SkASSERT(this->unique());
         return (T*)(this + 1);
     }
     T* writableEnd() {
         return this->writableBegin() + fCount;
     }
     T& writableAt(int index) {
         SkASSERT((unsigned)index < (unsigned)fCount);
         return this->writableBegin()[index];
     }
 
@@ skipping 15 matching lines @@
 private:
     int fCount;
 
     // hide this
     virtual ~SkTRefArray() {}
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif