Improve CancellableTaskFactory handling and Oilpan usage.

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 973 matching lines @@
         ASSERT(!m_keepAlive || m_keepAlive->get() == self);
         if (!m_keepAlive)
             m_keepAlive = adoptPtr(new Persistent<Self>);
         *m_keepAlive = self;
     }
 
     GC_PLUGIN_IGNORE("420515")
     OwnPtr<Persistent<Self>> m_keepAlive;
 };
 
+template<typename T>
+class AllowCrossThreadWeakPersistent {
+    STACK_ALLOCATED();
+public:
+    explicit AllowCrossThreadWeakPersistent(T* value) : m_value(value) { }
+    CrossThreadWeakPersistent<T> value() const { return m_value; }

[haraken, 2015/09/03 23:39:33]

Just to confirm: All call sites of value() are written assuming that it may
return 0, right?

[sof, 2015/09/04 05:21:04]

No, doing so would require teaching wtf/Functional.h about
blink::(CrossThread)WeakPersistent<>.

For CancellableTaskFactory, weakness is checked for & handled before the closure
is executed/applied.

[haraken, 2015/09/04 05:23:38]

Makes sense.

[sof, 2015/09/04 06:50:20]

Added a comment + TODO next to the relevant unwrap() here.

Having wtf/ abstractions work over types that are part of the Oilpan
infrastructure is becoming increasingly worthwhile.

+private:
+    CrossThreadWeakPersistent<T> m_value;
+};
+
 } // namespace blink
 
 namespace WTF {
 
 template <typename T> struct VectorTraits<blink::Member<T>> : VectorTraitsBase<blink::Member<T>> {
     static const bool needsDestruction = false;
     static const bool canInitializeWithMemset = true;
     static const bool canClearUnusedSlotsWithMemset = true;
     static const bool canMoveWithMemcpy = true;
 };
@@ skipping 171 matching lines @@
 struct ParamStorageTraits<T*> : public PointerParamStorageTraits<T*, blink::IsGarbageCollectedType<T>::value> {
     static_assert(sizeof(T), "T must be fully defined");
 };
 
 template<typename T>
 struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blink::IsGarbageCollectedType<T>::value> {
     static_assert(sizeof(T), "T must be fully defined");
 };
 
 template<typename T>
+struct ParamStorageTraits<blink::AllowCrossThreadWeakPersistent<T>> {
+    static_assert(sizeof(T), "T must be fully defined");
+    using StorageType = blink::CrossThreadWeakPersistent<T>;
+
+    static StorageType wrap(const blink::AllowCrossThreadWeakPersistent<T>& value) { return value.value(); }
+    static T* unwrap(const StorageType& value) { return value.get(); }
+};
+
+template<typename T>
 PassRefPtr<T> adoptRef(blink::RefCountedGarbageCollected<T>*) = delete;
 
 } // namespace WTF
 
 #endif