Add non-inline SkRTreeFactory constructor to fix clang-cl

src/core/SkBBHFactory.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBBHFactory.h"
 #include "SkRect.h"
 #include "SkRTree.h"
 #include "SkScalar.h"
 
 SkBBoxHierarchy* SkRTreeFactory::operator()(const SkRect& bounds) const {
     SkScalar aspectRatio = bounds.width() / bounds.height();
     return new SkRTree(aspectRatio);
 }
+
+// TODO(thakis@chromium): remove once HTTP://llvm.org/26506 is fixed
+SkRTreeFactory::SkRTreeFactory() {
+}