Adjust compositing tests to be SPv2-compatible.

third_party/WebKit/LayoutTests/compositing/overflow/resources/composited-scroll.js

+function isUsingCompositedScrolling(layers) {
+    var foundScrollingContentsLayer = false;
+    layers["layers"].forEach(function(layer) {
+        if (layer.name == "Scrolling Contents Layer")
+            foundScrollingContentsLayer = true;
+    });
+
+    return foundScrollingContentsLayer;
+}