[SVGDom] Add <svg> viewBox support

experimental/svg/model/SkSVGContainer.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSVGContainer.h"
 
 SkSVGContainer::SkSVGContainer(SkSVGTag t) : INHERITED(t) { }
 
 void SkSVGContainer::appendChild(sk_sp<SkSVGNode> node) {
     SkASSERT(node);
     fChildren.push_back(std::move(node));
 }
 
-void SkSVGContainer::onRender(SkCanvas* canvas, const SkSVGRenderContext& ctx) const {
+void SkSVGContainer::onRender(const SkSVGRenderContext& ctx) const {
     for (int i = 0; i < fChildren.count(); ++i) {
-        fChildren[i]->render(canvas, ctx);
+        fChildren[i]->render(ctx);
     }
 }