[bindings] Use |PostMessage| attribute for Client.postMessage to eliminate custom bindings

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 f4609f8a419de8d5641824a42ffca9b69d988cdd..7e1b5477cbd872d8acd133711f6291da621ae43b 100644
--- a/Source/modules/serviceworkers/Client.idl
+++ b/Source/modules/serviceworkers/Client.idl
@@ -13,5 +13,5 @@
     [RuntimeEnabled=ServiceWorkerClientAttributes] readonly attribute USVString url;
     [RuntimeEnabled=ServiceWorkerClientAttributes] readonly attribute ContextFrameType frameType;
     // 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);
+    [PostMessage, RaisesException, DeprecateAs=ServiceWorkerClientPostMessage, CallWith=ExecutionContext] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
 };