Implement enough JDWP functionality that we can attach jdb.

You can also try "classes" and "classpath", though anything else
deadlocks because we're suspended but jdb thinks we aren't. I don't
think that's a new bug with this patch, though, so I'll look at that
next.

Change-Id: I54456b6a7fe72642be696c66aa485dc0c8a7f913
diff --git a/src/thread.h b/src/thread.h
index 7b8c8b9..a4fd3bc 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -233,6 +233,8 @@
     return peer_;
   }
 
+  Object* GetThreadGroup() const;
+
   RuntimeStats* GetStats() {
     return &stats_;
   }