Revert of [reland] 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 8f0f10cbdfeb7f9f7a9b6c618451a2b73c298bb5..83d0fe131cc3f6483213efe054195579350ddd5e 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;