Remember and reuse derived map for external arrays

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 022636d70b83da1131610131efa6b502c7cc921b..1c60b84e0a973b2c7fc4336c14b01f40f8a51b18 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -1100,11 +1100,11 @@ void MarkCompactCollector::MarkDescriptorArray(
   ASSERT(contents->IsFixedArray());
   ASSERT(contents->length() >= 2);
   SetMark(contents);
-  // Contents contains (value, details) pairs.  If the details say that
-  // the type of descriptor is MAP_TRANSITION, CONSTANT_TRANSITION, or
-  // NULL_DESCRIPTOR, we don't mark the value as live.  Only for
-  // MAP_TRANSITION and CONSTANT_TRANSITION is the value an Object* (a
-  // Map*).
+  // Contents contains (value, details) pairs.  If the details say that the type
+  // of descriptor is MAP_TRANSITION, CONSTANT_TRANSITION,
+  // EXTERNAL_ARRAY_TRANSITION or NULL_DESCRIPTOR, we don't mark the value as
+  // live.  Only for MAP_TRANSITION, EXTERNAL_ARRAY_TRANSITION and
+  // CONSTANT_TRANSITION is the value an Object* (a Map*).
   for (int i = 0; i < contents->length(); i += 2) {
     // If the pair (value, details) at index i, i+1 is not
     // a transition or null descriptor, mark the value.