Initial implementation of stashed message ports, layout tests

LayoutTests/http/tests/serviceworker/chromium/resources/stashed-ports-echo-worker.js

+
+self.onmessage = function(e) {
+  var port = self.ports.add(e.data.name, e.data.port);
+};
+
+self.ports.onmessage = function(e) {
+  e.source.postMessage({name: e.source.name, data: e.data});
+};