Add memory pressure notification API

src/heap/incremental-marking-job.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_HEAP_INCREMENTAL_MARKING_JOB_H_
 #define V8_HEAP_INCREMENTAL_MARKING_JOB_H_
 
 #include "src/cancelable-task.h"
 
 namespace v8 {
@@ skipping 31 matching lines @@
         : CancelableTask(isolate), job_(job) {}
     static void Step(Heap* heap);
     // CancelableTask overrides.
     void RunInternal() override;
 
    private:
     IncrementalMarkingJob* job_;
   };
 
   // Delay of the delayed task.
-  static const int kDelayInSeconds = 5;
+  static const double kLongDelayInSeconds;
+  static const double kShortDelayInSeconds;
 
   IncrementalMarkingJob()
       : idle_task_pending_(false),
         delayed_task_pending_(false),
         made_progress_since_last_delayed_task_(false) {}
 
   bool ShouldForceMarkingStep() {
     return !made_progress_since_last_delayed_task_;
   }
 
   bool IdleTaskPending() { return idle_task_pending_; }
 
   void Start(Heap* heap);
 
   void NotifyIdleTask();
   void NotifyDelayedTask();
   void NotifyIdleTaskProgress();
   void ScheduleIdleTask(Heap* heap);
   void ScheduleDelayedTask(Heap* heap);
 
  private:
   bool idle_task_pending_;
   bool delayed_task_pending_;
   bool made_progress_since_last_delayed_task_;
 };
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_INCREMENTAL_MARKING_JOB_H_