Add {CancelableTaskManager} to handle {Cancelable} concurrent tasks.

src/cancelable-task.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/cancelable-task.h"
 
 #include "src/base/platform/platform.h"
 #include "src/isolate.h"
 
 namespace v8 {
 namespace internal {
 
 
 Cancelable::Cancelable(Isolate* isolate)
-    : isolate_(isolate), is_cancelled_(false) {
+    : isolate_(isolate), status_(kWaiting) {

[Hannes Payer (out of office), 2015/11/04 23:19:00]

atomic set?

   isolate->RegisterCancelableTask(this);
 }
 
 
 Cancelable::~Cancelable() {
-  if (!is_cancelled_) {
+  if (CanRun() || IsRunning() || IsCanceled()) {

[Hannes Payer (out of office), 2015/11/04 23:19:00]

Why would you CanRun here?

+    // Get exclusive access here to avoid a race with the isolate when calling
+    // {CancelForShutdown}.
     isolate_->RemoveCancelableTask(this);
   }
 }
 
 
 }  // namespace internal
 }  // namespace v8