Adds a scavenge GC pass to collect unmodified references

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 75939db8bb1d9c7503ac7357dd5a510ca326ff21..13aa46c1b370f3c3f84038916fb7a8d42ae102eb 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7529,6 +7529,15 @@ void Isolate::VisitHandlesForPartialDependence(
 }
 
 
+void Isolate::VisitWeakHandles(PersistentHandleVisitor* visitor) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  i::DisallowHeapAllocation no_allocation;
+  VisitorAdapter visitor_adapter(visitor);
+  isolate->global_handles()->IterateWeakRootsInNewSpaceWithClassIds(
+      &visitor_adapter);
+}
+
+
 String::Utf8Value::Utf8Value(v8::Local<v8::Value> obj)
     : str_(NULL), length_(0) {
   if (obj.IsEmpty()) return;