Service Worker: Add a warning message for Client.postMessage

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 fa335273391c3442393e3391ee9c9580735ac277..de8930dae14f802bee02ccf5a81a07a4e6f65933 100644
--- a/Source/modules/serviceworkers/Client.idl
+++ b/Source/modules/serviceworkers/Client.idl
@@ -11,5 +11,6 @@
     ImplementedAs=ServiceWorkerClient,
 ] interface Client {
     [RuntimeEnabled=ServiceWorkerClientAttributes] readonly attribute USVString url;
-    [Custom, RaisesException, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
+    // FIXME: Un-deprecate when the discussion on https://github.com/slightlyoff/ServiceWorker/issues/609 settles.
+    [Custom, RaisesException, DeprecateAs=ServiceWorkerClientPostMessage, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
 };