[platform] Add NumberOfAvailableBackgroundThreads

include/v8-platform.h

 // Copyright 2013 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_V8_PLATFORM_H_
 #define V8_V8_PLATFORM_H_
 
+#include <stddef.h>
 #include <stdint.h>
 
 namespace v8 {
 
 class Isolate;
 
 /**
  * A Task represents a unit of work.
  */
 class Task {
@@ skipping 31 matching lines @@
    * whether to execute the task on a dedicated thread.
    */
   enum ExpectedRuntime {
     kShortRunningTask,
     kLongRunningTask
   };
 
   virtual ~Platform() {}
 
   /**
+   * Gets the number of threads that are used to execute background tasks. Is
+   * used to estimate the number of tasks a work package should be split into.
+   * A return value of 0 means that there are no background threads available.
+   * Note that a value of 0 won't prohibit V8 from posting tasks using
+   * |CallOnBackgroundThread|.
+   */
+  virtual size_t NumberOfAvailableBackgroundThreads() { return 0; }
+
+  /**
    * Schedules a task to be invoked on a background thread. |expected_runtime|
    * indicates that the task will run a long time. The Platform implementation
    * takes ownership of |task|. There is no guarantee about order of execution
    * of tasks wrt order of scheduling, nor is there a guarantee about the
    * thread the task will be run on.
    */
   virtual void CallOnBackgroundThread(Task* task,
                                       ExpectedRuntime expected_runtime) = 0;
 
   /**
@@ skipping 83 matching lines @@
    * Sets the duration field of a COMPLETE trace event. It must be called with
    * the handle returned from AddTraceEvent().
    **/
   virtual void UpdateTraceEventDuration(const uint8_t* category_enabled_flag,
                                         const char* name, uint64_t handle) {}
 };
 
 }  // namespace v8
 
 #endif  // V8_V8_PLATFORM_H_