Onerror return value handling in workers fixed as per HTML5 spec.

LayoutTests/fast/workers/resources/worker-error-handler-workerglobalscope.js

+self.onerror = function(e){
+    postMessage("workerGlobalScopeErrorHandlerInvoked");
+    postMessage("DONE");
+    // We have cancelled this error event, hence it should
+    // not be propagated to the worker.onerror error handler.
+    return true;
+}
+
+self.onmessage= function(e){
+    foo.bar = 0;
+}