[SVGDom] Initial linear gradient support

experimental/svg/model/SkSVGDOM.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 SkSVGDOM_DEFINED
 #define SkSVGDOM_DEFINED
 
 #include "SkRefCnt.h"
 #include "SkSize.h"
+#include "SkSVGIDMapper.h"
 #include "SkTemplates.h"
 
 class SkCanvas;
 class SkDOM;
 class SkStream;
 class SkSVGNode;
 
 class SkSVGDOM : public SkRefCnt {
 public:
     SkSVGDOM(const SkSize& containerSize);
     ~SkSVGDOM() = default;
 
     static sk_sp<SkSVGDOM> MakeFromDOM(const SkDOM&, const SkSize& containerSize);
     static sk_sp<SkSVGDOM> MakeFromStream(SkStream&, const SkSize& containerSize);
 
     void setContainerSize(const SkSize&);
     void setRoot(sk_sp<SkSVGNode>);
 
     void render(SkCanvas*) const;
 
 private:
     SkSize           fContainerSize;
     sk_sp<SkSVGNode> fRoot;
+    SkSVGIDMapper    fIDMapper;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif // SkSVGDOM_DEFINED