IIRC, the UMA histogram macros don't work well with enum classes, so that might be the reason for using plain old enums there.

Ah you mean it requires an extra static_cast<int>() ? Doesn't seem too bad, we'd do that only in a single place, when calling the macro, right?

The UMA_HISTOGRAM_ENUMERATION macro has some static_asserts on the passed-in types, and I *thought* those triggered if you just passed in ints. But from looking at the code (https://cs.chromium.org/webrtc/src/base/metrics/histogram_macros_internal.h?type=cs&q=INTERNAL_HISTOGRAM_ENUMERATION_WITH_FLAG&l=140), that shouldn't be the case. So yes, if all that's required is a cast to int, then that's fine.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7fd744931c8b702cc7977f36bb88e3ddab2ce5ee

commit 7fd744931c8b702cc7977f36bb88e3ddab2ce5ee
Author: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Date: Thu Jan 11 01:32:33 2018

Switch ntp_snippets enums to be enum class.

This patch makes ContentSuggestionsNotification{Impression/Action/OptOut}
to enum class for better type safety.
Rename the enumerators.

BUG= 710254 

Signed-off-by: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Change-Id: I7f4a227b37dbc7dd1c6ab17a6ad0fb84344d35d9
Reviewed-on: https://chromium-review.googlesource.com/850655
Reviewed-by: Nicolas Dossou-Gbété <dgn@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528522}
[modify] https://crrev.com/7fd744931c8b702cc7977f36bb88e3ddab2ce5ee/chrome/browser/android/ntp/android_content_suggestions_notifier.cc
[modify] https://crrev.com/7fd744931c8b702cc7977f36bb88e3ddab2ce5ee/chrome/browser/android/ntp/content_suggestions_notifier_service.cc
[modify] https://crrev.com/7fd744931c8b702cc7977f36bb88e3ddab2ce5ee/chrome/browser/ntp_snippets/ntp_snippets_metrics.cc
[modify] https://crrev.com/7fd744931c8b702cc7977f36bb88e3ddab2ce5ee/chrome/browser/ntp_snippets/ntp_snippets_metrics.h
[modify] https://crrev.com/7fd744931c8b702cc7977f36bb88e3ddab2ce5ee/chrome/browser/ui/webui/snippets_internals_message_handler.cc

Query to find remaining non-class enums: https://cs.chromium.org/search/?q=f:ntp_snippets+enum%5Cs%5Cw%2B%5Cs%7B+lang:cc+-f:out&sq=package:chromium&type=cs

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6b8cc4b9cbf971a02cb723772ff295a387e4d5df

commit 6b8cc4b9cbf971a02cb723772ff295a387e4d5df
Author: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Date: Wed Mar 21 09:21:28 2018

Switch ntp_snippets enums to be enum class instead of plain c++ enum.

This patch makes HistogramCategories, ContentSuggestionsService::State,
RemoteSuggestion::ContentType, ReceivedMessageAction and TestType
to enum class instead of plain C++ enum for better type safety.

Bug:  710254 

Signed-off-by: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Change-Id: I40ff10809aa5e135b3ac9380ba54e97170eed02d
Reviewed-on: https://chromium-review.googlesource.com/970049
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544658}
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/chrome/browser/android/ntp/content_suggestions_notifier_service_unittest.cc
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/components/ntp_snippets/breaking_news/breaking_news_gcm_app_handler_unittest.cc
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/components/ntp_snippets/breaking_news/breaking_news_metrics.h
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/components/ntp_snippets/content_suggestions_metrics.cc
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/components/ntp_snippets/content_suggestions_service.h
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/components/ntp_snippets/remote/cached_image_fetcher_unittest.cc
[modify] https://crrev.com/6b8cc4b9cbf971a02cb723772ff295a387e4d5df/components/ntp_snippets/remote/remote_suggestion.h

I think this issue is fixed.

Yup, thanks!