[compositorworker] compositor proxy mutation updates underlying layers

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

-onmessage = function(msg) {
-    var proxy = msg.data;
-    postMessage({
-      opacity: proxy.supports("opacity"),
-      transform: proxy.supports("transform"),
-      proxy: proxy
-    });
+self.onmessage = function(msg) {
+  var proxy = msg.data;
+  postMessage({
+    opacity: proxy.supports("opacity"),
+    transform: proxy.supports("transform"),
+    proxy: proxy
+  });
 }