[heap] Remove special handling of map space in old-to-old pointer update.

Now that the layout descriptor is pretenured, we don't need atomic
accessor for the map space.

BUG=chromium:694255

Change-Id: I0ced8c04eaa61eec0f6a7b518ecba6413a691501
Reviewed-on: https://chromium-review.googlesource.com/595742Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47042}

src/heap/mark-compact.cc

@@ -4121,18 +4121,7 @@ class RememberedSetUpdatingItem : public UpdatingItem {
   }
 
   void UpdateUntypedPointers() {
-    // A map slot might point to new space and be required for iterating
-    // an object concurrently by another task. Hence, we need to update
-    // those slots using atomics.
     if (chunk_->slot_set<OLD_TO_NEW, AccessMode::NON_ATOMIC>() != nullptr) {
-      if (chunk_->owner() == heap_->map_space()) {
-        RememberedSet<OLD_TO_NEW>::Iterate(
-            chunk_,
-            [this](Address slot) {
-              return CheckAndUpdateOldToNewSlot<AccessMode::ATOMIC>(slot);
-            },
-            SlotSet::PREFREE_EMPTY_BUCKETS);
-      } else {
         RememberedSet<OLD_TO_NEW>::Iterate(
             chunk_,
             [this](Address slot) {
@@ -4140,18 +4129,8 @@ class RememberedSetUpdatingItem : public UpdatingItem {
             },
             SlotSet::PREFREE_EMPTY_BUCKETS);
     }
-    }
     if ((updating_mode_ == RememberedSetUpdatingMode::ALL) &&
         (chunk_->slot_set<OLD_TO_OLD, AccessMode::NON_ATOMIC>() != nullptr)) {
-      if (chunk_->owner() == heap_->map_space()) {
-        RememberedSet<OLD_TO_OLD>::Iterate(
-            chunk_,
-            [](Address slot) {
-              return UpdateSlot<AccessMode::ATOMIC>(
-                  reinterpret_cast<Object**>(slot));
-            },
-            SlotSet::PREFREE_EMPTY_BUCKETS);
-      } else {
         RememberedSet<OLD_TO_OLD>::Iterate(
             chunk_,
             [](Address slot) {
@@ -4161,7 +4140,6 @@ class RememberedSetUpdatingItem : public UpdatingItem {
             SlotSet::PREFREE_EMPTY_BUCKETS);
     }
   }
-  }
 
   void UpdateTypedPointers() {
     Isolate* isolate = heap_->isolate();