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

src/image/SkSurface.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Base.h"
 #include "SkImagePriv.h"
 #include "SkCanvas.h"
@@ skipping 32 matching lines @@
     if (NULL != fCachedCanvas) {
         SkASSERT(fCachedCanvas->getSurfaceBase() == this || \
                  NULL == fCachedCanvas->getSurfaceBase());
         fCachedCanvas->setSurfaceBase(NULL);
     }
 
     if (NULL != fCachedImage) {
         // the surface may need to fork its backend, if its sharing it with
         // the cached image. Note: we only call if there is an outstanding owner
         // on the image (besides us).
-        if (fCachedImage->getRefCnt() > 1) {
+        if (!fCachedImage->unique()) {
             this->onCopyOnWrite(mode);
         }
 
         // regardless of copy-on-write, we must drop our cached image now, so
         // that the next request will get our new contents.
         fCachedImage->unref();
         fCachedImage = NULL;
     }
 }
 
@@ skipping 38 matching lines @@
 }
 
 SkSurface* SkSurface::newSurface(const SkImage::Info& info) {
     return asSB(this)->onNewSurface(info);
 }
 
 void SkSurface::draw(SkCanvas* canvas, SkScalar x, SkScalar y,
                      const SkPaint* paint) {
     return asSB(this)->onDraw(canvas, x, y, paint);
 }