[compositorworker] Register compositor proxies with proxy client

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

 onmessage = function(msg) {
     var proxy = msg.data;
     if (typeof proxy == "CompositorProxy")

[jbroman, 2016/06/09 21:13:47]

By the way, what's this condition checking? How can 'typeof' ever return
"CompositorProxy" (rather than "object")?

[majidvp, 2016/06/10 15:33:47]

Done. + some other minor cleanups.

         postMessage({type: "error"});
     else
-        postMessage({type: 'response', opacity: proxy.supports('opacity'), transform: proxy.supports('transform')});
+        postMessage({
+          type: "response",
+          opacity: proxy.supports("opacity"),
+          transform: proxy.supports("transform"),
+          proxy: proxy
+        });
 }