Change how we report exceptions to the debugger.

This is only a refactoring/cleanup. Bug fixes with respect
to catch location, and more cleanups will follow.

Change-Id: I30d3c6260b0c8f8115a811621397225b88f2063a
diff --git a/runtime/mirror/throwable.h b/runtime/mirror/throwable.h
index f90812d..c22475b 100644
--- a/runtime/mirror/throwable.h
+++ b/runtime/mirror/throwable.h
@@ -51,6 +51,8 @@
     return java_lang_Throwable_.Read();
   }
 
+  int32_t GetStackDepth() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+
   static void SetClass(Class* java_lang_Throwable);
   static void ResetClass();
   static void VisitRoots(RootCallback* callback, void* arg)