Lazily initialize thread pool to avoid allocating all the threads during startup

src/libplatform/default-platform.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 41 matching lines @@
 
   // v8::Platform implementation.
   virtual void CallOnBackgroundThread(
       Task *task, ExpectedRuntime expected_runtime) V8_OVERRIDE;
   virtual void CallOnForegroundThread(v8::Isolate *isolate,
                                       Task *task) V8_OVERRIDE;
 
  private:
   static const int kMaxThreadPoolSize = 4;
 
+  void EnsureInitialized();
+
   Mutex lock_;
   bool initialized_;
+  int thread_pool_size_;
   std::vector<WorkerThread*> thread_pool_;
   TaskQueue queue_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultPlatform);
 };
 
 
 } }  // namespace v8::internal
 
 
 #endif  // V8_LIBPLATFORM_DEFAULT_PLATFORM_H_