Join embeder threads on Windows.

runtime/bin/dbg_connection.h

Index: runtime/bin/dbg_connection.h
diff --git a/runtime/bin/dbg_connection.h b/runtime/bin/dbg_connection.h
index bd4f4dd95f78e5858d7453012273324e638606fc..577c30a95cf92c881f8e7aa1708cbe55c25712b4 100644
--- a/runtime/bin/dbg_connection.h
+++ b/runtime/bin/dbg_connection.h
@@ -51,6 +51,9 @@ class DebuggerConnectionHandler {
   // from the client.
   static int StartHandler(const char* address, int port_number);
 
+  // Stops the native thread.
+  static void StopHandler();
+
   // Initializes the parts of the debugger which are needed by the vm
   // service.  This function should only be called when StartHandler
   // is not called.