Revert of Do not block in SimpleThread::Start().

content/renderer/categorized_worker_pool.h

Index: content/renderer/categorized_worker_pool.h
diff --git a/content/renderer/categorized_worker_pool.h b/content/renderer/categorized_worker_pool.h
index 0737b1ae47a6821a1fe73d6f5009eff891013274..cd3be4672b1be110014ca39574f7122862c940de 100644
--- a/content/renderer/categorized_worker_pool.h
+++ b/content/renderer/categorized_worker_pool.h
@@ -69,8 +69,9 @@
   // Create a new sequenced task graph runner.
   scoped_refptr<base::SequencedTaskRunner> CreateSequencedTaskRunner();
 
-  // Blocks until the background worker is running.
-  base::PlatformThreadId GetBackgroundWorkerThreadId() const;
+  base::PlatformThreadId background_worker_thread_id() const {
+    return threads_.back()->tid();
+  }
 
  protected:
   ~CategorizedWorkerPool() override;