[SVGDom] Add <svg> viewBox support

experimental/svg/model/SkSVGTransformableNode.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 "SkCanvas.h"
+#include "SkSVGRenderContext.h"
 #include "SkSVGTransformableNode.h"
 #include "SkSVGValue.h"
 
 SkSVGTransformableNode::SkSVGTransformableNode(SkSVGTag tag)
     : INHERITED(tag)
     , fMatrix(SkMatrix::I()) { }
 
+
+bool SkSVGTransformableNode::onPrepareToRender(SkSVGRenderContext* ctx) const {
+    if (!fMatrix.isIdentity()) {
+        ctx->canvas()->save();
+        ctx->canvas()->concat(fMatrix);
+    }
+
+    return this->INHERITED::onPrepareToRender(ctx);
+}
+
 void SkSVGTransformableNode::onSetAttribute(SkSVGAttribute attr, const SkSVGValue& v) {
     switch (attr) {
     case SkSVGAttribute::kTransform:
         if (const auto* transform = v.as<SkSVGTransformValue>()) {
             this->setTransform(*transform);
         }
         break;
     default:
         this->INHERITED::onSetAttribute(attr, v);
         break;
     }
 }