[SVGDom] Initial linear gradient support

experimental/svg/model/SkSVGDefs.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 SkSVGDefs_DEFINED
+#define SkSVGDefs_DEFINED
+
+#include "SkSVGHiddenContainer.h"
+
+class SkSVGDefs : public SkSVGHiddenContainer {
+public:
+    virtual ~SkSVGDefs() = default;
+    static sk_sp<SkSVGDefs> Make() { return sk_sp<SkSVGDefs>(new SkSVGDefs()); }
+
+private:
+    SkSVGDefs() : INHERITED(SkSVGTag::kDefs) {}
+
+    typedef SkSVGHiddenContainer INHERITED;
+};
+
+#endif // SkSVGDefs_DEFINED