Eliminate BrowserThread usage from PasswordStoreDefault.

chrome/browser/password_manager/password_store_mac.cc

Index: chrome/browser/password_manager/password_store_mac.cc
diff --git a/chrome/browser/password_manager/password_store_mac.cc b/chrome/browser/password_manager/password_store_mac.cc
index acde852f8e2143b49f4d94ce1ad74635514ee537..5f87ea656131ebf084e6a9cc8f8dab4b97e770c2 100644
--- a/chrome/browser/password_manager/password_store_mac.cc
+++ b/chrome/browser/password_manager/password_store_mac.cc
@@ -863,7 +863,7 @@ void PasswordStoreMac::ShutdownOnUIThread() {
 // arbitrarily long time (most notably, it can block on user confirmation
 // from a dialog). Run tasks on a dedicated thread to avoid blocking the DB
 // thread.
-scoped_refptr<base::SequencedTaskRunner>
+scoped_refptr<base::SingleThreadTaskRunner>
 PasswordStoreMac::GetBackgroundTaskRunner() {
   return (thread_.get()) ? thread_->message_loop_proxy() : NULL;
 }