Add comment to justify AllowDeferredHandleDereference in WeakCell factory.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index d2a8d79dcf9cb2ea84c72358dc645c8f1b1200e3..133f74d9d6d21d8cb9dc32de838aa4cd4f4b2101 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -942,6 +942,8 @@ Handle<PropertyCell> Factory::NewPropertyCell() {
 
 
 Handle<WeakCell> Factory::NewWeakCell(Handle<HeapObject> value) {
+  // It is safe to dereference the value because we are embedding it
+  // in cell and not inspecting its fields.
   AllowDeferredHandleDereference convert_to_cell;
   CALL_HEAP_FUNCTION(isolate(), isolate()->heap()->AllocateWeakCell(*value),
                      WeakCell);