Worklet: Introduce "pending tasks struct" concept defined in the Worklet spec

third_party/WebKit/Source/core/workers/WorkletPendingTasks.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef WorkletPendingTasks_h
+#define WorkletPendingTasks_h
+
+#include "bindings/core/v8/ScriptPromiseResolver.h"
+#include "platform/heap/Heap.h"
+
+namespace blink {
+
+// Implementation of the "pending tasks struct":
+// https://drafts.css-houdini.org/worklets/#pending-tasks-struct
+//
+// This also implements a part of the "fetch and invoke a worklet script"
+// algorithm:
+// https://drafts.css-houdini.org/worklets/#fetch-and-invoke-a-worklet-script
+//
+// All functions must be accessed on the main thread.
+class WorkletPendingTasks final : public GarbageCollected<WorkletPendingTasks> {
+ public:
+  WorkletPendingTasks(int counter, ScriptPromiseResolver*);
+
+  // Sets |counter_| to -1 and rejects the promise.
+  void Abort();
+
+  // Decrements |counter_| and resolves the promise if the counter becomes 0.
+  void DecrementCounter();
+
+  DEFINE_INLINE_VIRTUAL_TRACE() { visitor->Trace(resolver_); }
+
+ private:
+  // The number of pending tasks. -1 indicates these tasks are aborted and
+  // |resolver_| already rejected the promise.
+  int counter_;
+
+  Member<ScriptPromiseResolver> resolver_;
+};
+
+}  // namespace blink
+
+#endif  // WorkletPendingTasks_h