[map] Turn on in-place field representation changes again.

This switches --modify-field-representation-inplace to true again, after
the issue with the elements kinds transitions is fixed.

Cq-Include-Trybots: luci.chromium.try:linux-rel,win7-rel
Doc: http://bit.ly/v8-in-place-field-representation-changes
Bug: v8:8749, v8:8865, v8:9114, chromium:959645, chromium:952682
Change-Id: Ieb9a40d6ff44dd5f327e18ed7e7a169951c58416
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598705
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61291}

src/flag-definitions.h

@@ -1075,7 +1075,7 @@ DEFINE_GENERIC_IMPLICATION(
                   v8::tracing::TracingCategoryObserver::ENABLED_BY_NATIVE))
 DEFINE_BOOL_READONLY(fast_map_update, false,
                      "enable fast map update by caching the migration target")
-DEFINE_BOOL(modify_field_representation_inplace, false,
+DEFINE_BOOL(modify_field_representation_inplace, true,
             "enable in-place field representation updates")
 DEFINE_INT(max_polymorphic_map_count, 4,
            "maximum number of maps to track in POLYMORPHIC state")