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/thread_list.h b/src/thread_list.h
index d19622e..7bbd866 100644
--- a/src/thread_list.h
+++ b/src/thread_list.h
@@ -43,7 +43,7 @@
   void UndoDebuggerSuspensions();
 
   // Iterates over all the threads. The caller must hold the thread list lock.
-  void ForEach(void (*callback)(Thread*));
+  void ForEach(void (*callback)(Thread*, void*), void* context);
 
   void Register();
   void Unregister();