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

src/isolate.h

 // Copyright 2012 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.
 
 #ifndef V8_ISOLATE_H_
 #define V8_ISOLATE_H_
 
 #include <queue>
 #include <set>
 
@@ skipping 1325 matching lines @@
 
   v8::ArrayBuffer::Allocator* array_buffer_allocator_;
 
   FutexWaitListNode futex_wait_list_node_;
 
   std::set<Cancelable*> cancelable_tasks_;
 
   v8::Isolate::AbortOnUncaughtExceptionCallback
       abort_on_uncaught_exception_callback_;
 
+  base::Mutex isolate_mutex_;
+
   friend class ExecutionAccess;
   friend class HandleScopeImplementer;
   friend class OptimizingCompileDispatcher;
   friend class SweeperThread;
   friend class ThreadManager;
   friend class Simulator;
   friend class StackGuard;
   friend class ThreadId;
   friend class TestMemoryAllocatorScope;
   friend class TestCodeRangeScope;
@@ skipping 209 matching lines @@
 
   EmbeddedVector<char, 128> filename_;
   FILE* file_;
   int scope_depth_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ISOLATE_H_