Improve CancellableTaskFactory handling and Oilpan usage.

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 39046ada559b1e6a46e7c7d15ea4baa19a7743dc..5c7c4c68cbd111828faf1d926a5ee22bece309ad 100644
--- a/Source/platform/heap/Handle.h
+++ b/Source/platform/heap/Handle.h
@@ -991,6 +991,16 @@ private:
     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 {
@@ -1182,6 +1192,15 @@ struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blin
 };
 
 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