Retain maps that have live prototypes.

test/cctest/test-heap.cc

Index: test/cctest/test-heap.cc
diff --git a/test/cctest/test-heap.cc b/test/cctest/test-heap.cc
index d46d59a9128553f96d0c709cd22cd8c5c237d9e8..65a1214af84008d79f4fb37a577034c80d3239ce 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -5101,6 +5101,11 @@ void CheckMapRetainingFor(int n) {
   {
     HandleScope inner_scope(isolate);
     Handle<Map> map = Map::Create(isolate, 1);
+    v8::Local<v8::Value> result =
+        CompileRun("(function () { return {x : 10}; })();");
+    Handle<JSObject> proto =
+        v8::Utils::OpenHandle(*v8::Handle<v8::Object>::Cast(result));
+    map->set_prototype(*proto);
     heap->AddRetainedMap(map);
     weak_cell = inner_scope.CloseAndEscape(Map::WeakCellForMap(map));
   }