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

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 5abbc75df9f882e7b6af299c9bacbb6aa1c6ea0c..6a20c3e916633af6dca677d5cb22907da205ffa9 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1093,8 +1093,9 @@ class Isolate {
 
   FutexWaitListNode* futex_wait_list_node() { return &futex_wait_list_node_; }
 
-  void RegisterCancelableTask(Cancelable* task);
-  void RemoveCancelableTask(Cancelable* task);
+  CancelableTaskManager* cancelable_task_manager() {
+    return cancelable_task_manager_;
+  }
 
   interpreter::Interpreter* interpreter() const { return interpreter_; }
 
@@ -1338,7 +1339,7 @@ class Isolate {
 
   FutexWaitListNode futex_wait_list_node_;
 
-  std::set<Cancelable*> cancelable_tasks_;
+  CancelableTaskManager* cancelable_task_manager_;
 
   v8::Isolate::AbortOnUncaughtExceptionCallback
       abort_on_uncaught_exception_callback_;