Ship RegExp lookbehind assertions

Intent to ship:
https://groups.google.com/d/msg/v8-users/r-SN2yuKTL8/pfwrSuqSBQAJ

BUG=v8:4545

Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I15a8a5af211a3af1d2f952b48923288adc09d93c
Reviewed-on: https://chromium-review.googlesource.com/592705Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47024}

src/flag-definitions.h

@@ -210,7 +210,6 @@ DEFINE_IMPLICATION(es_staging, harmony)
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED(V)                                               \
   V(harmony_function_tostring, "harmony Function.prototype.toString")   \
-  V(harmony_regexp_lookbehind, "harmony regexp lookbehind")             \
   V(harmony_regexp_named_captures, "harmony regexp named captures")     \
   V(harmony_regexp_property, "harmony Unicode regexp property classes") \
   V(harmony_strict_legacy_accessor_builtins,                            \
@@ -228,7 +227,8 @@ DEFINE_IMPLICATION(es_staging, harmony)
     "harmony restrictions on generator declarations")                    \
   V(harmony_object_rest_spread, "harmony object rest spread properties") \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")              \
-  V(harmony_regexp_dotall, "harmony regexp dotAll flag")
+  V(harmony_regexp_dotall, "harmony regexp dotAll flag")                 \
+  V(harmony_regexp_lookbehind, "harmony regexp lookbehind")
 
 // 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,