[reland] 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 5569e06ac2edc0a3a9e46951297a8763b898a477..c13e5f4561169168ca17452a595d86ad78a825f7 100644
--- a/content/renderer/categorized_worker_pool.cc
+++ b/content/renderer/categorized_worker_pool.cc
@@ -257,6 +257,11 @@ CategorizedWorkerPool::CreateSequencedTaskRunner() {
   return new CategorizedWorkerPoolSequencedTaskRunner(this);
 }
 
+base::PlatformThreadId CategorizedWorkerPool::GetBackgroundWorkerThreadId()
+    const {
+  return threads_.back()->GetTid();
+}
+
 CategorizedWorkerPool::~CategorizedWorkerPool() {}
 
 cc::NamespaceToken CategorizedWorkerPool::GenerateNamespaceToken() {