Fix deadlock that can occur while handling service messages at a breakpoint.

runtime/vm/message_handler.h

Index: runtime/vm/message_handler.h
diff --git a/runtime/vm/message_handler.h b/runtime/vm/message_handler.h
index 65dc32f53c5597d5c0c563d6d57ec69debc34784..0d2cc540aebbce339e113743d7a0762faa249d92 100644
--- a/runtime/vm/message_handler.h
+++ b/runtime/vm/message_handler.h
@@ -54,6 +54,10 @@ class MessageHandler {
   // Returns true on success.
   bool HandleOOBMessages();
 
+  // Returns true if there are pending OOB messages for this message
+  // handler.
+  bool HasOOBMessages();
+
   // A message handler tracks how many live ports it has.
   bool HasLivePorts() const { return live_ports_ > 0; }