It was a regression from https://chromium-review.googlesource.com/c/chromium/src/+/1325393, which removed the relevant #include.

Working on a fix now.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/29ced3fcef80063c8d3feef7932ddd6ab08fbd5e

commit 29ced3fcef80063c8d3feef7932ddd6ab08fbd5e
Author: Kyle Horimoto <khorimoto@chromium.org>
Date: Tue Nov 20 00:05:06 2018

[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/1343350
Reviewed-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}
[modify] https://crrev.com/29ced3fcef80063c8d3feef7932ddd6ab08fbd5e/chromeos/components/proximity_auth/webui/proximity_auth_webui_handler.cc

Actually, I'll leave this open for now. I'll look at adding a test which ensures that this does not regress.

Fixed by https://chromium-review.googlesource.com/c/chromium/src/+/1362214, which defines chromeos::multidevice::SoftwareFeature with an operator<<().