Document CrossThreadWeakPersistent.

Source/platform/heap/Handle.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 321 matching lines @@
     CrossThreadPersistent(T& raw) : Parent(raw) { }
     CrossThreadPersistent(const CrossThreadPersistent& other) : Parent(other) { }
     template<typename U>
     CrossThreadPersistent(const CrossThreadPersistent<U>& other) : Parent(other) { }
     template<typename U>
     CrossThreadPersistent(const Member<U>& other) : Parent(other) { }
     template<typename U>
     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;
 public:
     CrossThreadWeakPersistent() : Parent() { }
     CrossThreadWeakPersistent(std::nullptr_t) : Parent(nullptr) { }
     CrossThreadWeakPersistent(T* raw) : Parent(raw) { }
     CrossThreadWeakPersistent(T& raw) : Parent(raw) { }
     CrossThreadWeakPersistent(const CrossThreadWeakPersistent& other) : Parent(other) { }
     template<typename U>
@@ skipping 827 matching lines @@
 struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blink::IsGarbageCollectedType<T>::value> {
     static_assert(sizeof(T), "T must be fully defined");
 };
 
 template<typename T>
 PassRefPtr<T> adoptRef(blink::RefCountedGarbageCollected<T>*) = delete;
 
 } // namespace WTF
 
 #endif