Remove unused include in sequenced_worker_pool.h

components/browser_sync/signin_confirmation_helper.h

Index: components/browser_sync/signin_confirmation_helper.h
diff --git a/components/browser_sync/signin_confirmation_helper.h b/components/browser_sync/signin_confirmation_helper.h
index 27bd62ef1bf02c1a293f0cc464c5509c8a173267..c006653df91703f2e1b4fbdd77a0b9dc2697e231 100644
--- a/components/browser_sync/signin_confirmation_helper.h
+++ b/components/browser_sync/signin_confirmation_helper.h
@@ -11,6 +11,10 @@
 #include "base/memory/ref_counted.h"
 #include "base/task/cancelable_task_tracker.h"
 
+namespace base {
+class SingleThreadTaskRunner;
+}
+
 namespace history {
 class HistoryService;
 class QueryResults;