Oilpan: provide a weak 'this' pointer abstraction for cancellable closures.

third_party/WebKit/Source/platform/heap/Handle.h

Index: third_party/WebKit/Source/platform/heap/Handle.h
diff --git a/third_party/WebKit/Source/platform/heap/Handle.h b/third_party/WebKit/Source/platform/heap/Handle.h
index 49d3f33e2e717c08c92c3d0f119052c99b777240..2d13ec28386c20ab95e99039aa8605963437d981 100644
--- a/third_party/WebKit/Source/platform/heap/Handle.h
+++ b/third_party/WebKit/Source/platform/heap/Handle.h
@@ -1299,11 +1299,17 @@ private:
     Persistent<Self> m_keepAlive;
 };
 
+// Only a very reduced form of weak heap object references can currently be held
+// by WTF::Closure<>s. i.e., bound as a 'this' pointer only.
+//
+// TODO(sof): once wtf/Functional.h is able to work over platform/heap/ types
+// (like CrossThreadWeakPersistent<>), drop the restriction on weak persistent
+// use by function closures (and rename this ad-hoc type.)
 template<typename T>
-class AllowCrossThreadWeakPersistent {
+class CrossThreadWeakPersistentThisPointer {
     STACK_ALLOCATED();
 public:
-    explicit AllowCrossThreadWeakPersistent(T* value) : m_value(value) { }
+    explicit CrossThreadWeakPersistentThisPointer(T* value) : m_value(value) { }
     CrossThreadWeakPersistent<T> value() const { return m_value; }
 private:
     CrossThreadWeakPersistent<T> m_value;
@@ -1569,15 +1575,18 @@ struct ParamStorageTraits<RawPtr<T>> : public PointerParamStorageTraits<T*, blin
 };
 
 template<typename T>
-struct ParamStorageTraits<blink::AllowCrossThreadWeakPersistent<T>> {
+struct ParamStorageTraits<blink::CrossThreadWeakPersistentThisPointer<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 StorageType wrap(const blink::CrossThreadWeakPersistentThisPointer<T>& value) { return value.value(); }
 
-    // Currently assume that the call sites of this unwrap() account for cleared weak references also.
-    // TODO(sof): extend WTF::FunctionWrapper call overloading to also handle (CrossThread)WeakPersistent.
-    static T* unwrap(const StorageType& value) { return value.get(); }
+    // WTF::FunctionWrapper<> handles WeakPtr<>, so recast this weak persistent
+    // into it.
+    //
+    // TODO(sof): remove this hack once wtf/Functional.h can also work with a type like
+    // CrossThreadWeakPersistent<>.
+    static WeakPtr<T> unwrap(const StorageType& value) { return WeakPtr<T>(WeakReference<T>::create(value.get())); }
 };
 
 template<typename T>