[SVGDom] Add <svg> viewBox support

experimental/svg/model/SkSVGContainer.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 SkSVGContainer_DEFINED
 #define SkSVGContainer_DEFINED
 
 #include "SkSVGTransformableNode.h"
 #include "SkTArray.h"
 
 class SkSVGContainer : public SkSVGTransformableNode {
 public:
     virtual ~SkSVGContainer() = default;
 
     void appendChild(sk_sp<SkSVGNode>) override;
 
 protected:
     SkSVGContainer(SkSVGTag);
 
-    void onRender(SkCanvas*, const SkSVGRenderContext&) const override;
+    void onRender(const SkSVGRenderContext&) const override;
 
 private:
     SkSTArray<1, sk_sp<SkSVGNode>, true> fChildren;
 
     typedef SkSVGTransformableNode INHERITED;
 };
 
 #endif // SkSVGSVG_DEFINED