PPAPI: Add unlocking for PPP calls and callbacks. Add more locking.

ppapi/proxy/ppp_messaging_proxy.cc

Index: ppapi/proxy/ppp_messaging_proxy.cc
diff --git a/ppapi/proxy/ppp_messaging_proxy.cc b/ppapi/proxy/ppp_messaging_proxy.cc
index 7a45fd30a3a147fddf7f3cb646fa6b5f32357443..8a35705a51051e036181c413861d784dbd1d3dcd 100644
--- a/ppapi/proxy/ppp_messaging_proxy.cc
+++ b/ppapi/proxy/ppp_messaging_proxy.cc
@@ -13,6 +13,7 @@
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/serialized_var.h"
 #include "ppapi/shared_impl/ppapi_globals.h"
+#include "ppapi/shared_impl/proxy_lock.h"
 #include "ppapi/shared_impl/var_tracker.h"
 
 namespace ppapi {
@@ -85,7 +86,9 @@ void PPP_Messaging_Proxy::OnMsgHandleMessage(
   // SerializedVarReceiveInput will decrement the reference count, but we want
   // to give the recipient a reference.
   PpapiGlobals::Get()->GetVarTracker()->AddRefVar(received_var);
-  ppp_messaging_impl_->HandleMessage(instance, received_var);
+  CallWhileUnlocked(ppp_messaging_impl_->HandleMessage,
+                    instance,
+                    received_var);
 }
 
 }  // namespace proxy