Add support for linear gradients, implemented as skia shaders.

I had to complicate the IDL bindings generation to allow passing an array of
colors. Without these changes, we'd try to convert the dart object to
Vector<SkColor>, which C++ thinks is Vector<unsigned>, and we'd use the wrong
converter. So I added some template grease to force it to use a
Vector<CanvasColor> converter.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1152963009

examples/raw/painting.sky

@@ -26,8 +26,12 @@ void main() {
     paint.color = const Color.fromARGB(128, 255, 0, 255);
     context.rotateDegrees(45.0);
 
+    Gradient yellowBlue = new Gradient.Linear(
+        [new Point(-radius, -radius), new Point(0.0, 0.0)],
+        [const Color(0xFFFFFF00), const Color(0xFF0000FF)],
+        null);
     context.drawRect(new Rect.fromLTRB(-radius, -radius, radius, radius),
-                     paint);
+                     new Paint()..setShader(yellowBlue));
 
     // Scale x and y by 0.5.
     var scaleMatrix = new Float32List.fromList([