Hook up CSSPaintValue::image to CSS Paint API callback.

third_party/WebKit/LayoutTests/csspaint/resources/paint-logging-green.js

+// Logs the size of the geometry to the console.
+// Fills the context with a green square.
+
+registerPaint('logging-green', class {
+    paint(ctx, geom) {
+        console.log('width: ' + geom.width + ', height: ' + geom.height);
+        ctx.fillStyle = 'green';
+        ctx.fillRect(0, 0, geom.width, geom.height);
+    }
+});