[compositorworker] compositor proxy mutation updates underlying layers

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

+self.importScripts('worker-common.js');
+
+self.onmessage = function(msg) {
+  if (msg.data.proxy)
+    self.proxy = msg.data.proxy;
+
+  awaitProxyInit(self.proxy).then((proxy) => {
+    var keys = ['opacity', 'scrollTop', 'scrollLeft'];
+    var values = {};
+    for (var i = 0; i < keys.length; i++) {
+      values[keys[i]] = proxy[keys[i]];
+    }
+    values.transform = proxy.transform.toFloat32Array();
+    postMessage(values);
+  });
+}