allow SkAutoTUnref to be used syntactially as the object is refs

Review URL: https://codereview.appspot.com/6189059

git-svn-id: http://skia.googlecode.com/svn/trunk@3870 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkRefCnt.h

@@ -128,6 +128,9 @@ public:
         return obj;
     }
 
+    T* operator->() { return fObj; }
+    operator T*() { return fObj; }
+
 private:
     T*  fObj;
 };