Jan suggested making boundary width multiplicative (i.e. based on actual category count), this may eliminate need for top categories at all (they will have more clicks, so the boundary will be larger by default). We need to consider this option. My only concern is that then we control the behavior less (currently I know all boundary widths, but then I won't) and we may miss some cases where it works badly.

CL
https://codereview.chromium.org/2599293002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1e8492aea8959ffaf818bb05f28368e524505ab7

commit 1e8492aea8959ffaf818bb05f28368e524505ab7
Author: vitaliii <vitaliii@chromium.org>
Date: Wed Jan 04 10:53:13 2017

[NTP::SectionOrder] Implement click counts decay.

The click based ranker must "forget" history with time, so that changes
in the user behavior are reflected by the order in reasonable time. This
is done in thic CL using multiplicative click count decay with time.

Count overflow is ignored (it is unlikely to happen due to the decay) and a comment is added.

BUG= 675918 , 676279 

Review-Url: https://codereview.chromium.org/2599293002
Cr-Commit-Position: refs/heads/master@{#441353}

[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/category_rankers/click_based_category_ranker.cc
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/category_rankers/click_based_category_ranker.h
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/category_rankers/click_based_category_ranker_unittest.cc
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/features.cc
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/features.h
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/pref_names.cc
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/components/ntp_snippets/pref_names.h
[modify] https://crrev.com/1e8492aea8959ffaf818bb05f28368e524505ab7/ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc