[es6] ship @@toStringTag

BUG=v8:3502
R=arv@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1072573002

Cr-Commit-Position: refs/heads/master@{#27767}

src/flag-definitions.h

@@ -197,14 +197,14 @@ DEFINE_IMPLICATION(es_staging, harmony)
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                               \
-  V(harmony_tostring, "harmony toString")                               \
   V(harmony_unicode, "harmony unicode escapes")
 
 // Features that are shipping (turned on by default, but internal flag remains).
 #define HARMONY_SHIPPING(V)                                                \
   V(harmony_classes, "harmony classes (implies object literal extension)") \
   V(harmony_computed_property_names, "harmony computed property names")    \
-  V(harmony_object_literals, "harmony object literal extensions")
+  V(harmony_object_literals, "harmony object literal extensions")          \
+  V(harmony_tostring, "harmony toString")
 
 // Once a shipping feature has proved stable in the wild, it will be dropped
 // from HARMONY_SHIPPING, all occurrences of the FLAG_ variable are removed,