Initial scroll-blocks-on compositor integration

LayoutTests/compositing/layer-creation/scroll-blocks-on-explicit-expected.txt

+Verifies that setting the scroll-blocks-on property in a stylesheet creates layers with the correct modes and that explicitly setting 'none' does not create a layer.
+{
+  "bounds": [800, 600],
+  "children": [
+    {
+      "bounds": [800, 600],
+      "scrollBlocksOn": ["StartTouch", "WheelEvent"],
+      "contentsOpaque": true,
+      "drawsContent": true,
+      "children": [
+        {
+          "position": [8, 48],
+          "bounds": [784, 1],
+          "scrollBlocksOn": ["WheelEvent"]
+        },
+        {
+          "position": [8, 49],
+          "bounds": [784, 2],
+          "scrollBlocksOn": ["StartTouch", "ScrollEvent"]
+        }
+      ]
+    }
+  ]
+}
+