Document CrossThreadWeakPersistent.

Source/platform/heap/Handle.h

Index: Source/platform/heap/Handle.h
diff --git a/Source/platform/heap/Handle.h b/Source/platform/heap/Handle.h
index 69db16effb787d17e80c31311c28c44c09985621..39046ada559b1e6a46e7c7d15ea4baa19a7743dc 100644
--- a/Source/platform/heap/Handle.h
+++ b/Source/platform/heap/Handle.h
@@ -339,6 +339,7 @@ public:
     CrossThreadPersistent(const RawPtr<U>& other) : Parent(other.get()) { }
 };
 
+// Combines the behavior of CrossThreadPersistent and WeakPersistent.
 template<typename T>
 class CrossThreadWeakPersistent : public PersistentBase<T, WeakPersistentConfiguration, CrossThreadPersistentConfiguration> {
     typedef PersistentBase<T, WeakPersistentConfiguration, CrossThreadPersistentConfiguration> Parent;