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

sky/engine/core/painting/CanvasGradient.cpp

Index: sky/engine/core/painting/CanvasGradient.cpp
diff --git a/sky/engine/core/painting/CanvasGradient.cpp b/sky/engine/core/painting/CanvasGradient.cpp
index 5ceaae149883336a51b08da8a9d848fff4b6b101..32d9dfd143f1e4956f9ffb1a9d771df71a8bbe2b 100644
--- a/sky/engine/core/painting/CanvasGradient.cpp
+++ b/sky/engine/core/painting/CanvasGradient.cpp
@@ -14,7 +14,7 @@ PassRefPtr<CanvasGradient> CanvasGradient::create() {
 void CanvasGradient::initLinear(const Vector<Point>& end_points,
                                 const Vector<SkColor>& colors,
                                 const Vector<float>& color_stops,
-                                unsigned tile_mode) {
+                                SkShader::TileMode tile_mode) {
   ASSERT(end_points.size() == 2);
   ASSERT(colors.size() == color_stops.size() || color_stops.data() == nullptr);
   ASSERT(tile_mode < SkShader::kTileModeCount);
@@ -25,7 +25,7 @@ void CanvasGradient::initLinear(const Vector<Point>& end_points,
   // TODO(mpcomplete): allow setting the TileMode.
   SkShader* shader = SkGradientShader::CreateLinear(
       sk_end_points, colors.data(), color_stops.data(), colors.size(),
-      static_cast<SkShader::TileMode>(tile_mode));
+      tile_mode);
   set_shader(adoptRef(shader));
 }
 
@@ -33,13 +33,13 @@ void CanvasGradient::initRadial(const Point& center,
                                 double radius,
                                 const Vector<SkColor>& colors,
                                 const Vector<float>& color_stops,
-                                unsigned tile_mode) {
+                                SkShader::TileMode tile_mode) {
   ASSERT(colors.size() == color_stops.size() || color_stops.data() == nullptr);
   ASSERT(tile_mode < SkShader::kTileModeCount);
 
   SkShader* shader = SkGradientShader::CreateRadial(
       center.sk_point, radius, colors.data(), color_stops.data(), colors.size(),
-      static_cast<SkShader::TileMode>(tile_mode));
+      tile_mode);
   set_shader(adoptRef(shader));
 }