Support calling BackgroundSyncProviderThreadProxy::GetThreadInstance after the worker has stopped

content/child/background_sync/background_sync_provider_thread_proxy.cc

Index: content/child/background_sync/background_sync_provider_thread_proxy.cc
diff --git a/content/child/background_sync/background_sync_provider_thread_proxy.cc b/content/child/background_sync/background_sync_provider_thread_proxy.cc
index 90584bfc8380a6d31a2280ddd33303e83e52439a..97466b23bf18dc84ee0787d8d4c817ea9fda156a 100644
--- a/content/child/background_sync/background_sync_provider_thread_proxy.cc
+++ b/content/child/background_sync/background_sync_provider_thread_proxy.cc
@@ -62,6 +62,7 @@ LazyInstance<ThreadLocalPointer<BackgroundSyncProviderThreadProxy>>::Leaky
 
 }  // anonymous namespace
 
+// static
 BackgroundSyncProviderThreadProxy*
 BackgroundSyncProviderThreadProxy::GetThreadInstance(
     base::SingleThreadTaskRunner* main_thread_task_runner,
@@ -69,10 +70,15 @@ BackgroundSyncProviderThreadProxy::GetThreadInstance(
   if (g_sync_provider_tls.Pointer()->Get())
     return g_sync_provider_tls.Pointer()->Get();
 
+  if (!WorkerTaskRunner::Instance()->CurrentWorkerId()) {
+    // This could happen if GetThreadInstance is called very late (say by a
+    // garbage collected SyncRegistration).
+    return nullptr;
+  }
+
   BackgroundSyncProviderThreadProxy* instance =
       new BackgroundSyncProviderThreadProxy(main_thread_task_runner,
                                             sync_provider);
-  DCHECK(WorkerTaskRunner::Instance()->CurrentWorkerId());
   WorkerTaskRunner::Instance()->AddStopObserver(instance);
   return instance;
 }