Explicitly initialize Code::builtin_index

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index a1a52b6eee75ef4c97542aedafc29f5e99f347b6..901e389a68b6707f1694a1883fa343f54c7122d5 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5170,11 +5170,9 @@ class Code: public HeapObject {
   inline int profiler_ticks();
   inline void set_profiler_ticks(int ticks);
 
-  // [builtin_index]: For BUILTIN kind, tells which builtin index it has.
-  // For builtins, tells which builtin index it has.
-  // Note that builtins can have a code kind other than BUILTIN, which means
-  // that for arbitrary code objects, this index value may be random garbage.
-  // To verify in that case, compare the code object to the indexed builtin.
+  // [builtin_index]: For builtins, tells which builtin index the code object
+  // has. Note that builtins can have a code kind other than BUILTIN. The
+  // builtin index is a non-negative integer for builtins, and -1 otherwise.
   inline int builtin_index();
   inline void set_builtin_index(int id);