Make object groups and implicit references a bit more lightweight.

src/profile-generator.cc

Index: src/profile-generator.cc
diff --git a/src/profile-generator.cc b/src/profile-generator.cc
index fd3268d4f64e89d83c06a0f2e0ea2ae61e6593aa..4cf62e256d28a60dfab5af12c6a4d6b4b6019437 100644
--- a/src/profile-generator.cc
+++ b/src/profile-generator.cc
@@ -2295,7 +2295,7 @@ void NativeObjectsExplorer::FillRetainedObjects() {
     ObjectGroup* group = groups->at(i);
     if (group->info_ == NULL) continue;
     List<HeapObject*>* list = GetListMaybeDisposeInfo(group->info_);
-    for (int j = 0; j < group->objects_.length(); ++j) {
+    for (size_t j = 0; j < group->length_; ++j) {
       HeapObject* obj = HeapObject::cast(*group->objects_[j]);
       list->Add(obj);
       in_groups_.Insert(obj);