[CrOS MultiDevice] Fix broken chrome://proximity-auth page.

This CL fixes a regression which replaced feature names on this page
with numbers representing the features. The regression was caused by
erroneous removal of the #include containing the operator<<() function
which prints the enum names.

Bug: 906836
Change-Id: Idb35e491ff379cc0ea97e63b57b4078a85efd0af
Reviewed-on: https://chromium-review.googlesource.com/c/1343350Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609516}

chromeos/components/proximity_auth/webui/proximity_auth_webui_handler.cc

@@ -21,6 +21,7 @@
 #include "chromeos/components/proximity_auth/messenger.h"
 #include "chromeos/components/proximity_auth/remote_device_life_cycle_impl.h"
 #include "chromeos/components/proximity_auth/remote_status_update.h"
+#include "components/cryptauth/proto/enum_util.h"
 #include "components/cryptauth/software_feature_state.h"
 #include "components/prefs/pref_service.h"
 #include "content/public/browser/browser_thread.h"