[SVGDom] Initial linear gradient support

experimental/svg/model/SkSVGValue.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 SkSVGValue_DEFINED
 #define SkSVGValue_DEFINED
 
 #include "SkColor.h"
 #include "SkMatrix.h"
 #include "SkPath.h"
 #include "SkSVGTypes.h"
 #include "SkTypes.h"
 
 class SkSVGValue : public SkNoncopyable {
 public:
     enum class Type {
         kColor,
         kLength,
         kLineCap,
         kLineJoin,
         kNumber,
         kPaint,
         kPath,
         kPoints,
+        kString,
         kTransform,
         kViewBox,
     };
 
     Type type() const { return fType; }
 
     template <typename T>
     const T* as() const {
         return fType == T::TYPE ? static_cast<const T*>(this) : nullptr;
     }
@@ skipping 32 matching lines @@
 using SkSVGColorValue     = SkSVGWrapperValue<SkSVGColorType    , SkSVGValue::Type::kColor    >;
 using SkSVGLengthValue    = SkSVGWrapperValue<SkSVGLength       , SkSVGValue::Type::kLength   >;
 using SkSVGPathValue      = SkSVGWrapperValue<SkPath            , SkSVGValue::Type::kPath     >;
 using SkSVGTransformValue = SkSVGWrapperValue<SkSVGTransformType, SkSVGValue::Type::kTransform>;
 using SkSVGViewBoxValue   = SkSVGWrapperValue<SkSVGViewBoxType  , SkSVGValue::Type::kViewBox  >;
 using SkSVGPaintValue     = SkSVGWrapperValue<SkSVGPaint        , SkSVGValue::Type::kPaint    >;
 using SkSVGLineCapValue   = SkSVGWrapperValue<SkSVGLineCap      , SkSVGValue::Type::kLineCap  >;
 using SkSVGLineJoinValue  = SkSVGWrapperValue<SkSVGLineJoin     , SkSVGValue::Type::kLineJoin >;
 using SkSVGNumberValue    = SkSVGWrapperValue<SkSVGNumberType   , SkSVGValue::Type::kNumber   >;
 using SkSVGPointsValue    = SkSVGWrapperValue<SkSVGPointsType   , SkSVGValue::Type::kPoints   >;
+using SkSVGStringValue    = SkSVGWrapperValue<SkSVGStringType   , SkSVGValue::Type::kString   >;
 
 #endif // SkSVGValue_DEFINED