Track changes to the layout of java.lang.Throwable.

The stack trace is now stored in an object called |backtrace|.

Change-Id: Icb3f43fed7bc13ecd9124b0e012fef33b7cdbe50
diff --git a/runtime/mirror/throwable.h b/runtime/mirror/throwable.h
index f90812d..ddfb721 100644
--- a/runtime/mirror/throwable.h
+++ b/runtime/mirror/throwable.h
@@ -58,16 +58,16 @@
 
  private:
   Object* GetStackState() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
-    return GetFieldObjectVolatile<Object>(OFFSET_OF_OBJECT_MEMBER(Throwable, stack_state_));
+    return GetFieldObjectVolatile<Object>(OFFSET_OF_OBJECT_MEMBER(Throwable, backtrace_));
   }
   Object* GetStackTrace() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     return GetFieldObjectVolatile<Object>(OFFSET_OF_OBJECT_MEMBER(Throwable, stack_trace_));
   }
 
   // Field order required by test "ValidateFieldOrderOfJavaCppUnionClasses".
+  HeapReference<Object> backtrace_;  // Note this is Java volatile:
   HeapReference<Throwable> cause_;
   HeapReference<String> detail_message_;
-  HeapReference<Object> stack_state_;  // Note this is Java volatile:
   HeapReference<Object> stack_trace_;
   HeapReference<Object> suppressed_exceptions_;