[platform] Add NumberOfAvailableBackgroundThreads

src/libplatform/default-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_LIBPLATFORM_DEFAULT_PLATFORM_H_
 #define V8_LIBPLATFORM_DEFAULT_PLATFORM_H_
 
 #include <functional>
 #include <map>
 #include <queue>
@@ skipping 16 matching lines @@
   DefaultPlatform();
   virtual ~DefaultPlatform();
 
   void SetThreadPoolSize(int thread_pool_size);
 
   void EnsureInitialized();
 
   bool PumpMessageLoop(v8::Isolate* isolate);
 
   // v8::Platform implementation.
+  size_t NumberOfAvailableBackgroundThreads() override;
   void CallOnBackgroundThread(Task* task,
                               ExpectedRuntime expected_runtime) override;
   void CallOnForegroundThread(v8::Isolate* isolate, Task* task) override;
   void CallDelayedOnForegroundThread(Isolate* isolate, Task* task,
                                      double delay_in_seconds) override;
   void CallIdleOnForegroundThread(Isolate* isolate, IdleTask* task) override;
   bool IdleTasksEnabled(Isolate* isolate) override;
   double MonotonicallyIncreasingTime() override;
   const uint8_t* GetCategoryGroupEnabled(const char* name) override;
   const char* GetCategoryGroupName(
@@ skipping 28 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(DefaultPlatform);
 };
 
 
 }  // namespace platform
 }  // namespace v8
 
 
 #endif  // V8_LIBPLATFORM_DEFAULT_PLATFORM_H_