[compositorworker] compositor proxy mutation updates underlying layers

third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/visual-update.js

+self.onmessage = function(e) {
+  self.proxy = e.data.proxy;
+  var update = function(timestamp) {
+    if (!self.proxy.initialized) {
+      requestAnimationFrame(update);
+      return;
+    }
+    self.proxy.opacity = 0.5;
+    var transform = self.proxy.transform;
+    transform.m42 = 100.0;
+    self.proxy.transform = transform;
+    self.proxy.scrollLeft = 100;
+    self.proxy.scrollTop = 100;
+    self.postMessage({});
+  };
+  requestAnimationFrame(update);
+}