Add non-inline SkRTreeFactory constructor to fix clang-cl

include/core/SkBBHFactory.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkBBHFactory_DEFINED
 #define SkBBHFactory_DEFINED
 
 #include "SkTypes.h"
 class SkBBoxHierarchy;
 struct SkRect;
 
 class SK_API SkBBHFactory {
 public:
     /**
      *  Allocate a new SkBBoxHierarchy. Return NULL on failure.
      */
     virtual SkBBoxHierarchy* operator()(const SkRect& bounds) const = 0;
     virtual ~SkBBHFactory() {};
 };
 
 class SK_API SkRTreeFactory : public SkBBHFactory {
 public:
+    SkRTreeFactory();
     SkBBoxHierarchy* operator()(const SkRect& bounds) const override;
 private:
     typedef SkBBHFactory INHERITED;
 };
 
 #endif