[SVGDom] Add <svg> viewBox support

experimental/svg/model/SkSVGNode.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 SkSVGNode_DEFINED
 #define SkSVGNode_DEFINED
 
@@ skipping 13 matching lines @@
 };
 
 class SkSVGNode : public SkRefCnt {
 public:
     virtual ~SkSVGNode();
 
     SkSVGTag tag() const { return fTag; }
 
     virtual void appendChild(sk_sp<SkSVGNode>) = 0;
 
-    void render(SkCanvas*, const SkSVGRenderContext&) const;
+    void render(const SkSVGRenderContext&) const;
 
     void setAttribute(SkSVGAttribute, const SkSVGValue&);
 
 protected:
     SkSVGNode(SkSVGTag);
 
-    virtual void onRender(SkCanvas*, const SkSVGRenderContext&) const = 0;
+    // Called before onRender(), to apply local attributes to the context.  Unlike onRender(),
+    // onPrepareToRender() bubbles up the inheritance chain: overriders should always call
+    // INHERITED::onPrepareToRender(), unless they intend to short-circuit rendering
+    // (return false).
+    // Implementations are expected to return true if rendering is to continue, or false if
+    // the node/subtree rendering is disabled.
+    virtual bool onPrepareToRender(SkSVGRenderContext*) const;
+
+    virtual void onRender(const SkSVGRenderContext&) const = 0;
 
     virtual void onSetAttribute(SkSVGAttribute, const SkSVGValue&);
 
-    virtual const SkMatrix& onLocalMatrix() const;
-
 private:
     SkSVGTag                    fTag;
 
     // FIXME: this should be sparse
     SkSVGPresentationAttributes fPresentationAttributes;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif // SkSVGNode_DEFINED