Implement debugger support for the "threads" command.

Change-Id: I266d28ee00d87fdc7ecb179ddf52634db96e573c
diff --git a/src/debugger.h b/src/debugger.h
index ca1d2a0..ca39316 100644
--- a/src/debugger.h
+++ b/src/debugger.h
@@ -178,7 +178,7 @@
    */
   static bool GetThreadName(JDWP::ObjectId threadId, std::string& name);
   static JDWP::ObjectId GetThreadGroup(JDWP::ObjectId threadId);
-  static char* GetThreadGroupName(JDWP::ObjectId threadGroupId);
+  static std::string GetThreadGroupName(JDWP::ObjectId threadGroupId);
   static JDWP::ObjectId GetThreadGroupParent(JDWP::ObjectId threadGroupId);
   static JDWP::ObjectId GetSystemThreadGroupId();
   static JDWP::ObjectId GetMainThreadGroupId();