Cache multiple ElementsKind map transition per map.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 2378a5e0a4e0690ab99a5701125bc9d331d846c4..54a293685f2875b2a8935b6586b0cfe0788acc18 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -1626,13 +1626,23 @@ void MarkCompactCollector::MarkDescriptorArray(
 
     RecordSlot(slot, slot, *slot);
 
-    if (details.type() < FIRST_PHANTOM_PROPERTY_TYPE) {
+    PropertyType type = details.type();
+    if (type < FIRST_PHANTOM_PROPERTY_TYPE) {
       HeapObject* object = HeapObject::cast(value);
       MarkBit mark = Marking::MarkBitFrom(HeapObject::cast(object));
       if (!mark.Get()) {
         SetMark(HeapObject::cast(object), mark);
         marking_deque_.PushBlack(object);
       }
+    } else if (type == ELEMENTS_TRANSITION && value->IsFixedArray()) {
+      // For maps with multiple elements transitions, the transition maps are
+      // stored in a FixedArray. Keep the fixed array alive but not the maps
+      // that it refers to.
+      HeapObject* object = HeapObject::cast(value);
+      MarkBit mark = Marking::MarkBitFrom(HeapObject::cast(object));
+      if (!mark.Get()) {
+        SetMark(HeapObject::cast(object), mark);
+      }
     }
   }
   // The DescriptorArray descriptors contains a pointer to its contents array,