Bug 792652 - Remove methods from MessageListener (r=dvander)

With this change, MessageChannel stores mListener as an IToplevelProtocol
rather than a MessageListener (which isn't really a useful concept on
its own). The MessageListener methods are split out to IProtocol and
IToplevelProtocol. MessageListener gets deleted. Some of the inline
functions in MessageChannel had to be moved to MessageChannel.cpp since
IToplevelProtocol isn't defined in MessageChannel.h.

dom/plugins/ipc/PluginModuleParent.cpp

@@ -1036,8 +1036,7 @@ PluginInstanceParent*
 PluginModuleChromeParent::GetManagingInstance(mozilla::ipc::IProtocol* aProtocol)
 {
     MOZ_ASSERT(aProtocol);
-    mozilla::ipc::MessageListener* listener =
-        static_cast<mozilla::ipc::MessageListener*>(aProtocol);
+    mozilla::ipc::IProtocol* listener = aProtocol;
     switch (listener->GetProtocolTypeId()) {
         case PPluginInstanceMsgStart:
             // In this case, aProtocol is the instance itself. Just cast it.