[SVGDom] Add <svg> viewBox support

experimental/svg/model/SkSVGTransformableNode.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSVGTransformableNode_DEFINED
 #define SkSVGTransformableNode_DEFINED
 
 #include "SkMatrix.h"
 #include "SkSVGNode.h"
 
 class SkSVGTransformableNode : public SkSVGNode {
 public:
     virtual ~SkSVGTransformableNode() = default;
 
     void setTransform(const SkMatrix& m) { fMatrix = m; }
 
 protected:
     SkSVGTransformableNode(SkSVGTag);
 
+    bool onPrepareToRender(SkSVGRenderContext*) const override;
+
     void onSetAttribute(SkSVGAttribute, const SkSVGValue&) override;
 
-    const SkMatrix& onLocalMatrix() const override { return fMatrix; }
-
 private:
     // FIXME: should be sparse
     SkMatrix fMatrix;
 
     typedef SkSVGNode INHERITED;
 };
 
 #endif // SkSVGTransformableNode_DEFINED