buffet: Add flag to disable XMPP support

As a temporary work around to major source of flake in tests, allow
disabling XMPP support in tests.

BUG=brillo:769
TEST=FEATURES="test" USE="clang asan" emerge-gizmo buffet && \
     FEATURES="test" emerge-gizmo buffet

Change-Id: I41ca92bb77b77132c1a20c7018b0ce737b69cf60
Reviewed-on: https://chromium-review.googlesource.com/264392
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
diff --git a/buffet/manager.h b/buffet/manager.h
index f44e075..d21f062 100644
--- a/buffet/manager.h
+++ b/buffet/manager.h
@@ -51,6 +51,7 @@
       const base::FilePath& config_path,
       const base::FilePath& state_path,
       const base::FilePath& test_definitions_path,
+      bool xmpp_enabled,
       const chromeos::dbus_utils::AsyncEventSequencer::CompletionAction& cb);
 
  private: