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

src/cancelable-task.cc

Index: src/cancelable-task.cc
diff --git a/src/cancelable-task.cc b/src/cancelable-task.cc
index 5927c22cdeb20b18081b4cdcdacbc6c3977ee72e..3413d459abe692d659e7ed320e3616e988d6f984 100644
--- a/src/cancelable-task.cc
+++ b/src/cancelable-task.cc
@@ -12,13 +12,15 @@ 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);
   }
 }