[Extensions] Add an external extensions XWalkExtensionClient to RendererController

We also add a Initialize(IPC::Sender*) method to XWalkExtensionClient,
so we can create a XWalkExtensionClient, initialize a SyncChannel using
the Client as its listener and only after initialize the Client by passing
the SyncChannel to be its Sender. This change follows exactly how
XWalkExtensionServer is designed nowadays.