Sky: Add a DartConverterEnum and use that for all our enum needs.

sky/engine/core/painting/PaintingStyle.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_CORE_PAINTING_PAINTINGSTYLE_H_
 #define SKY_ENGINE_CORE_PAINTING_PAINTINGSTYLE_H_
 
-#include "sky/engine/core/painting/Rect.h"
-#include "sky/engine/tonic/dart_wrappable.h"
-#include "sky/engine/wtf/PassRefPtr.h"
-#include "sky/engine/wtf/RefCounted.h"
+#include "sky/engine/tonic/dart_converter.h"
 #include "third_party/skia/include/core/SkPaint.h"
 
 namespace blink {
 
 class PaintingStyle {};
 
 template <>
-struct DartConverter<PaintingStyle> {
-  static SkPaint::Style FromArgumentsWithNullCheck(Dart_NativeArguments args,
-                                                   int index,
-                                                   Dart_Handle& exception);
+struct DartConverter<PaintingStyle> : public DartConverterEnum<SkPaint::Style> {
 };
 
+// If this fails, it's because SkPaint::Style has changed. We need to change
+// PaintingStyle.dart to ensure the PaintingStyle enum is in sync with the C++
+// values.
+COMPILE_ASSERT(SkPaint::kStyleCount == 3, Need_to_update_PaintingStyle_dart);
+
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_PAINTINGSTYLE_H_