Follow-up crrev.com/555893 use std::string instead of auto

Bug: 833834
Change-Id: Iea9d32052c6ffbe17bf2cdd5075cbf5e9a203e0a
Reviewed-on: https://chromium-review.googlesource.com/1042428Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555932}

chrome/browser/chromeos/file_manager/file_manager_browsertest_base.cc

@@ -768,7 +768,7 @@ void FileManagerBrowserTestBase::OnCommand(const std::string& name,
     ASSERT_TRUE(value.GetString("extensionId", &extension_id));
     ASSERT_TRUE(value.GetString("notificationId", &notification_id));
 
-    const auto delegate_id = extension_id + "-" + notification_id;
+    const std::string delegate_id = extension_id + "-" + notification_id;
     base::Optional<message_center::Notification> notification =
         display_service_->GetNotification(delegate_id);
     EXPECT_TRUE(notification);