Replace fill_color and stroke_color with fill_style and stroke_style in CanvasContextState.

The fillStyle and strokeStyle attributes can be either
strings(color), CanvasGradients, or CanvasPatterns.

components/script/dom/canvasgradient.rs

@@ -22,6 +22,7 @@ pub struct CanvasGradient {
 }
 
 #[jstraceable]
+#[derive(Clone)]
 pub enum CanvasGradientStyle {
     Linear(LinearGradientStyle),
     Radial(RadialGradientStyle),