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

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

Index: LayoutTests/fast/workers/resources/worker-error-handler-workerglobalscope.js
diff --git a/LayoutTests/fast/workers/resources/worker-error-handler-workerglobalscope.js b/LayoutTests/fast/workers/resources/worker-error-handler-workerglobalscope.js
new file mode 100644
index 0000000000000000000000000000000000000000..d923577d03d300c5b697852870935e4ba24f3d28
--- /dev/null
+++ b/LayoutTests/fast/workers/resources/worker-error-handler-workerglobalscope.js
@@ -0,0 +1,11 @@
+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;
+}