[compositorworker] compositor proxy mutation updates underlying layers

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

+self.importScripts('worker-common.js');
+
+self.onmessage = function(msg) {
+  if (msg.data.proxy)
+    self.proxy = msg.data.proxy;
+
+  awaitProxyInit(self.proxy).then((proxy) => {
+    proxy.opacity = 0.5;
+    var transform = proxy.transform;
+    transform.m41 = 30.0;
+    transform.m44 = 2;
+    proxy.transform = transform;
+    proxy.scrollLeft = 10;
+    proxy.scrollTop = 20;
+    postMessage({});
+  });
+}