Expose and use constants for DrawLooperLayerInfo.setPaintBits to dart.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1175403002.

examples/raw/painting.sky

@@ -51,7 +51,7 @@ void main() {
             new DrawLooperLayerInfo()
               ..setOffset(const Point(150.0, 0.0))
               ..setColorMode(TransferMode.srcMode)
-              ..setPaintBits(-1),
+              ..setPaintBits(PaintBits.all),
             (Paint layerPaint) {
           layerPaint.color = const Color.fromARGB(128, 255, 255, 0);
           layerPaint.setColorFilter(
@@ -64,13 +64,16 @@ void main() {
             new DrawLooperLayerInfo()
               ..setOffset(const Point(75.0, 75.0))
               ..setColorMode(TransferMode.srcMode)
-              ..setPaintBits(-1),
+              ..setPaintBits(PaintBits.shader),
             (Paint layerPaint) {
           Gradient redYellow = new Gradient.radial(
               new Point(0.0, 0.0), radius/3.0,
               [const Color(0xFFFFFF00), const Color(0xFFFF0000)],
               null, TileMode.mirror);
           layerPaint.setShader(redYellow);
+          // Since we're don't set PaintBits.maskFilter, this has no effect.
+          layerPaint.setMaskFilter(
+              new MaskFilter.blur(BlurStyle.normal, 50.0, highQuality: true));
         })
         ..addLayerOnTop(
             new DrawLooperLayerInfo()..setOffset(const Point(225.0, 75.0)),

examples/raw/shadow.dart

@@ -15,7 +15,7 @@ void beginFrame(double timeStamp) {
     // Shadow layer.
     ..addLayerOnTop(
         new DrawLooperLayerInfo()
-          ..setPaintBits(-1)
+          ..setPaintBits(PaintBits.all)
           ..setOffset(const Point(5.0, 5.0))
           ..setColorMode(TransferMode.srcInMode),
         (Paint layerPaint) {