[compositorworker] compositor proxy mutation updates underlying layers

third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/basic-plumbing-worker-to-main.js

+ self.onmessage = function(e) {
+  if (e.data.proxy)
+    self.proxy = e.data.proxy;
+  var respond = function(timestamp) {
+    if (!self.proxy.initialized) {
+      requestAnimationFrame(respond);
+      return;
+    }
+    self.proxy.opacity = 0.5;
+    var transform = self.proxy.transform;
+    transform.m41 = 30.0;
+    transform.m44 = 2;
+    self.proxy.transform = transform;
+    self.proxy.scrollLeft = 10;
+    self.proxy.scrollTop = 20;
+    self.postMessage({});
+  };
+  requestAnimationFrame(respond);
+}