[compositorworker] compositor proxy mutation updates underlying layers

third_party/WebKit/LayoutTests/virtual/threaded/fast/compositorworker/resources/proxy-disconnect.js

-onmessage = function(event) {
-    var proxy = event.data;
+self.importScripts('worker-common.js');
+
+self.onmessage = function(msg) {
+  awaitProxyInit(msg.data).then((proxy) => {
     proxy.opacity;
     proxy.disconnect();
     try {
-        proxy.opacity;
+      proxy.opacity;
+      postMessage("Able to read mutable property after disconnect.");
     } catch (e) {
-        postMessage(e.name);
+      postMessage(e.name);
     }
+  });
 }