Makes objects known to the debugger GC roots, implements the THST message, and lets DDMS request stack traces.

This fills out correct data in all columns of the "Threads" table, and
double-clicking on a thread shows that thread's stack.

Change-Id: I48f63c3612e12d35269158dc3a283f07db28c8e7
diff --git a/src/debugger.h b/src/debugger.h
index 111282c..270b62a 100644
--- a/src/debugger.h
+++ b/src/debugger.h
@@ -116,6 +116,8 @@
   // The debugger wants the VM to exit.
   static void Exit(int status);
 
+  static void VisitRoots(Heap::RootVisitor* visitor, void* arg);
+
   /*
    * Class, Object, Array
    */