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 79988f8f27a9a0c45f7d66fd4c2df9b46c4b81f0..a7f169848d8d6506a2c89ab86f94b4380beadca4 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_;