Ship the relative indexing method `.at`

I2S with LGTMs:
https://groups.google.com/a/chromium.org/g/blink-dev/c/I8S78w7aFmE/m/qLHAcjhRCQAJ

v8: 10961
Change-Id: If0440c0595823c61352f144c1fc29c54a1175623
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2574716Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71619}

src/flags/flag-definitions.h

@@ -264,9 +264,8 @@ DEFINE_IMPLICATION(harmony_weak_refs_with_cleanup_some, harmony_weak_refs)
 #endif
 
 // Features that are complete (but still behind --harmony/es-staging flag).
-#define HARMONY_STAGED_BASE(V)                          \
-  V(harmony_top_level_await, "harmony top level await") \
-  V(harmony_relative_indexing_methods, "harmony relative indexing methods")
+#define HARMONY_STAGED_BASE(V) \
+  V(harmony_top_level_await, "harmony top level await")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_STAGED(V)               \
@@ -278,13 +277,14 @@ DEFINE_IMPLICATION(harmony_weak_refs_with_cleanup_some, harmony_weak_refs)
 #endif
 
 // Features that are shipping (turned on by default, but internal flag remains).
-#define HARMONY_SHIPPING_BASE(V)                                          \
-  V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")               \
-  V(harmony_atomics, "harmony atomics")                                   \
-  V(harmony_weak_refs, "harmony weak references")                         \
-  V(harmony_string_replaceall, "harmony String.prototype.replaceAll")     \
-  V(harmony_logical_assignment, "harmony logical assignment")             \
-  V(harmony_atomics_waitasync, "harmony Atomics.waitAsync")
+#define HARMONY_SHIPPING_BASE(V)                                      \
+  V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")           \
+  V(harmony_atomics, "harmony atomics")                               \
+  V(harmony_weak_refs, "harmony weak references")                     \
+  V(harmony_string_replaceall, "harmony String.prototype.replaceAll") \
+  V(harmony_logical_assignment, "harmony logical assignment")         \
+  V(harmony_atomics_waitasync, "harmony Atomics.waitAsync")           \
+  V(harmony_relative_indexing_methods, "harmony relative indexing methods")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_SHIPPING(V) \