[platform] Add NumberOfAvailableBackgroundThreads

src/libplatform/default-platform.cc

 // 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.
 
 #include "src/libplatform/default-platform.h"
 
 #include <algorithm>
 #include <queue>
 
 #include "src/base/logging.h"
@@ skipping 175 matching lines @@
   static uint8_t no = 0;
   return &no;
 }
 
 
 const char* DefaultPlatform::GetCategoryGroupName(
     const uint8_t* category_enabled_flag) {
   static const char dummy[] = "dummy";
   return dummy;
 }
+
+size_t DefaultPlatform::NumberOfAvailableBackgroundThreads() {
+  return static_cast<size_t>(thread_pool_size_);
+}
+
 }  // namespace platform
 }  // namespace v8