Add a comment about map collection into MarkCompactCollector::MarkUnmarkedObject.

Review URL: http://codereview.chromium.org/6992059

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8056 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mark-compact.cc

@@ -1060,6 +1060,11 @@ void MarkCompactCollector::MarkUnmarkedObject(HeapObject* object) {
       map->ClearCodeCache(heap());
     }
     SetMark(map);
+
+    // When map collection is enabled we have to mark through map's transitions
+    // in a special way to make transition links weak.
+    // Only maps with instance types between FIRST_JS_OBJECT_TYPE and
+    // JS_FUNCTION_TYPE can have transitions.
     if (FLAG_collect_maps &&
         map->instance_type() >= FIRST_JS_OBJECT_TYPE &&
         map->instance_type() <= JS_FUNCTION_TYPE) {