Add waitable event for SyncMessageFilter init

ipc/ipc_sync_message_filter.h

Index: ipc/ipc_sync_message_filter.h
diff --git a/ipc/ipc_sync_message_filter.h b/ipc/ipc_sync_message_filter.h
index ee6677ef31011185dcb1b2d59c4d92a4e586a956..32dffe39d3a4894a34887452769ac329e2cfff3d 100644
--- a/ipc/ipc_sync_message_filter.h
+++ b/ipc/ipc_sync_message_filter.h
@@ -9,6 +9,7 @@
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_vector.h"
 #include "base/synchronization/lock.h"
 #include "ipc/ipc_sender.h"
 #include "ipc/ipc_sync_message.h"
@@ -59,6 +60,9 @@ class IPC_EXPORT SyncMessageFilter : public MessageFilter, public Sender {
   typedef std::set<PendingSyncMsg*> PendingSyncMessages;
   PendingSyncMessages pending_sync_messages_;
 
+  // Messages waiting to be delivered after IO initialization.
+  ScopedVector<Message> pending_messages_;
+
   // Locks data members above.
   base::Lock lock_;