PPAPI: Implement synchronous postMessage

BUG=367896

Review URL: https://codereview.chromium.org/264303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278102 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/ppapi/proxy/ppp_messaging_proxy.h b/ppapi/proxy/ppp_messaging_proxy.h
index 210574ab..d9dde3cb 100644
--- a/ppapi/proxy/ppp_messaging_proxy.h
+++ b/ppapi/proxy/ppp_messaging_proxy.h
@@ -27,6 +27,9 @@
   // Message handlers.
   void OnMsgHandleMessage(PP_Instance instance,
                           SerializedVarReceiveInput data);
+  void OnMsgHandleBlockingMessage(PP_Instance instance,
+                                  SerializedVarReceiveInput data,
+                                  IPC::Message* reply);
 
   // When this proxy is in the plugin side, this value caches the interface
   // pointer so we don't have to retrieve it from the dispatcher each time.