Add blink-side binding code and tests for ServiceWorker -> Document postMessage (3/3)

Source/modules/serviceworkers/Client.idl

Index: Source/modules/serviceworkers/Client.idl
diff --git a/Source/modules/serviceworkers/Client.idl b/Source/modules/serviceworkers/Client.idl
index 8e419c09a200d9f83ff4917b068f7d32585d18a8..ea1fbe34b1e70ff0fd631b9d3ed20a8439dcc9b7 100644
--- a/Source/modules/serviceworkers/Client.idl
+++ b/Source/modules/serviceworkers/Client.idl
@@ -7,4 +7,9 @@
     RuntimeEnabled=ServiceWorker
 ] interface Client {
     readonly attribute unsigned long id;
+
+    // FIXME: Currently we think targetOrigin's probably not necessary, but
+    // re-add it if the ongoing issue [1] concludes the other way.
+    // [1] https://github.com/slightlyoff/ServiceWorker/issues/190
+    [Custom, RaisesException, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional MessagePort[] messagePorts);
 };