Enso ThreadGroup.groups is an array not ArrayList

ThreadGroup.groups in the enso branch uses plain
array in place of ArrayList. debugger.cc code
needed minor adjustments to access it.

Bug: 25857261
Change-Id: I77ffb92b64a3642933c3240dbadedab5def5dc1b
diff --git a/runtime/well_known_classes.h b/runtime/well_known_classes.h
index c856291..55158a7 100644
--- a/runtime/well_known_classes.h
+++ b/runtime/well_known_classes.h
@@ -153,6 +153,7 @@
   static jfieldID java_lang_Thread_uncaughtHandler;
   static jfieldID java_lang_Thread_nativePeer;
   static jfieldID java_lang_ThreadGroup_groups;
+  static jfieldID java_lang_ThreadGroup_ngroups;
   static jfieldID java_lang_ThreadGroup_mainThreadGroup;
   static jfieldID java_lang_ThreadGroup_name;
   static jfieldID java_lang_ThreadGroup_parent;