Historically, V8Platform::CallOnBackgroundThread() posted tasks to base::WorkerPool. When a task is posted to base::WorkerPool, it is scheduled on an idle thread (if any) or on a new thread. That means that an unbounded number of threads can be created as a result of calling V8Platform::CallOnBackgroundThread(). To avoid this explosion, v8 limits the number of pending V8Platform::CallOnBackgroundThread() tasks to V8Platform::NumberOfAvailableBackgroundThreads().

Now that V8Platform::CallOnBackgroundThread() posts tasks to base::TaskScheduler, which has a configurable maximum number of threads, V8Platform::NumberOfAvailableBackgroundThreads() could be removed.