[compositorworker] compositor proxy mutation updates underlying layers

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

-onmessage = function(msg) {
-    var proxy = msg.data[0];
-    var attrib = msg.data[1];
+self.importScripts('worker-common.js');
+
+self.onmessage = function(msg) {
+  var proxy = msg.data[0];
+  var attrib = msg.data[1];
+
+  awaitProxyInit(proxy).then((proxy) => {
     try {
-        if (proxy.supports(attrib)) {
-            proxy[attrib] = 0;
-            postMessage('success');
-        } else {
-            postMessage('error: Received non-supported attribute "' + attrib + '"');
-        }
+      if (proxy.supports(attrib)) {
+        proxy[attrib] = 0;
+        postMessage('success');
+      } else {
+        postMessage('error: Received non-supported attribute "' + attrib + '"');
+      }
     } catch (e) {
-        postMessage('error: ' + e);
+      postMessage('error: ' + e);
     }
+  });
 }