[optional-chaining] Ship optional chaining.

Blink-dev intent to ship thread:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/TI4h1Gj5TYE

Bug: v8:9553
Change-Id: Icef9e4aeec1e519ab8ce11109e2529c61437a10b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1895427
Commit-Queue: Joshua Litt <joshualitt@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64715}

src/flags/flag-definitions.h

@@ -217,7 +217,6 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED_BASE(V)                                     \
-  V(harmony_optional_chaining, "harmony optional chaining syntax") \
   V(harmony_private_methods, "harmony private methods in class literals")
 
 #ifdef V8_INTL_SUPPORT
@@ -240,7 +239,8 @@ DEFINE_IMPLICATION(harmony_import_meta, harmony_dynamic_import)
   V(harmony_import_meta, "harmony import.meta property")       \
   V(harmony_dynamic_import, "harmony dynamic import")          \
   V(harmony_promise_all_settled, "harmony Promise.allSettled") \
-  V(harmony_nullish, "harmony nullish operator")
+  V(harmony_nullish, "harmony nullish operator")               \
+  V(harmony_optional_chaining, "harmony optional chaining syntax")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_SHIPPING(V)                               \