Add option to force verifiable parent message transport [#2]

[johnfargo]

Reviewers: shindig.remailer, beaton,

Description:
Once more, with feeling. Continuation of 28115.

Please review this at http://codereview.appspot.com/32052

Affected files:
  features/src/main/javascript/features/rpc/rpc.js


Index: features/src/main/javascript/features/rpc/rpc.js
===================================================================
--- features/src/main/javascript/features/rpc/rpc.js    (revision 758850)
+++ features/src/main/javascript/features/rpc/rpc.js    (working copy)
@@ -783,6 +783,19 @@
     },

     /**
+     * Forces all subsequent calls to be made by a transport
+     * method that allows the caller to verify the message receiver
+     * (by way of the parent parameter, through getRelayUrl(...)).
+     * At present this means IFPC only, since WPM/DPM targetOrigin
+     * is not yet validated.
+     */
+    forceParentVerifiable: function() {
+      if (relayChannel !== 'wpm') {
+        relayChannel = 'ifpc';
+      }
+    },
+
+    /**
      * Calls an RPC service.
      * @param {String} targetId Module Id of the RPC service provider.
      *                          Empty if calling the parent container.