Do not block in SimpleThread::Start().

content/renderer/categorized_worker_pool.cc

Index: content/renderer/categorized_worker_pool.cc
diff --git a/content/renderer/categorized_worker_pool.cc b/content/renderer/categorized_worker_pool.cc
index 8873ad196bab77bfbdd912d430a6d6ad73dc3258..9d0e8cf45c04e1a92010ca9c9ae5665279e15cb0 100644
--- a/content/renderer/categorized_worker_pool.cc
+++ b/content/renderer/categorized_worker_pool.cc
@@ -256,6 +256,11 @@ CategorizedWorkerPool::CreateSequencedTaskRunner() {
   return new CategorizedWorkerPoolSequencedTaskRunner(this);
 }
 
+base::PlatformThreadId CategorizedWorkerPool::GetBackgroundWorkerThreadId()
+    const {
+  return threads_.back()->GetTid();
+}
+
 CategorizedWorkerPool::~CategorizedWorkerPool() {}
 
 cc::NamespaceToken CategorizedWorkerPool::GenerateNamespaceToken() {