Move (uppercase) JS builtins from js builtins object to native context.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index e2187be3f9f05ee48de99971ee8aaba47366d466..d416fbad8342a5dba48c937d29d902b34c0aabab 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -658,10 +658,6 @@ class Isolate {
     return context()->global_proxy();
   }
 
-  Handle<JSBuiltinsObject> js_builtins_object() {
-    return Handle<JSBuiltinsObject>(thread_local_top_.context_->builtins());
-  }
-
   static int ArchiveSpacePerThread() { return sizeof(ThreadLocalTop); }
   void FreeThreadResources() { thread_local_top_.Free(); }