Sky: Allow clients to specify tile mode for gradients (repeating or mirror).

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1166223004.

examples/raw/painting.sky

@@ -68,9 +68,9 @@ void main() {
               ..setPaintBits(-1),
             (Paint layerPaint) {
           Gradient redYellow = new Gradient.Radial(
-              new Point(0.0, 0.0), radius,
+              new Point(0.0, 0.0), radius/3.0,
               [const Color(0xFFFFFF00), const Color(0xFFFF0000)],
-              null);
+              null, TileMode.mirror);
           layerPaint.setShader(redYellow);
         })
         ..addLayerOnTop(