Fixed build issue introduced with last commit.

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
diff --git a/runtime/vm/thread.cc b/runtime/vm/thread.cc
index c91500be28d31e3e72ff043ba8e76a5a33fb4f88..765ed26213dbaa22efa19cea31c7f4e60decbc10 100644
--- a/runtime/vm/thread.cc
+++ b/runtime/vm/thread.cc
@@ -211,8 +211,8 @@ void Thread::InitVMConstants() {
 void Thread::PrintJSON(JSONStream* stream) const {
   JSONObject jsobj(stream);
   jsobj.AddProperty("type", "_Thread");
-  jsobj.AddPropertyF("id", "threads/%" Pd64 "",
-                     ThreadIdToIntPtr(os_thread()->trace_id()));
+  jsobj.AddPropertyF("id", "threads/%" Pd "",
+                     OSThread::ThreadIdToIntPtr(os_thread()->trace_id()));
   Zone* zone = zone_;
   {
     JSONArray zone_info_array(&jsobj, "zones");