The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/454a93cffb64135a92ba1d8b282be7d1d45f07df

commit 454a93cffb64135a92ba1d8b282be7d1d45f07df
Author: justincohen <justincohen@chromium.org>
Date: Wed Feb 22 18:48:02 2017

[ios] Add new ntp histogram type.

BUG= 694808 

Review-Url: https://codereview.chromium.org/2707113006
Cr-Commit-Position: refs/heads/master@{#452145}

[modify] https://crrev.com/454a93cffb64135a92ba1d8b282be7d1d45f07df/ios/chrome/browser/metrics/new_tab_page_uma.h

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/ios_internal.git/+/37381242c0f9568804163d0430791ce75dba0d1f

commit 37381242c0f9568804163d0430791ce75dba0d1f
Author: justincohen <justincohen@google.com>
Date: Wed Feb 22 20:07:21 2017

Should this be in M58 or M59 by now ? It doesn't look like it according to this histogram:
https://uma.googleplex.com/p/chrome/histograms/?endDate=20170426&dayCount=1&histograms=NewTabPage.ActioniOS&fixupData=true&showMax=true&filters=platform%2Ceq%2Ci%2Cchannel%2Ceq%2C4%2Csimple_version%2Ceq%2C58.0.3029.83%2Cisofficial%2Ceq%2CTrue&implicitFilters=isofficial

It's UserActions.MobileNTPLogoTapped (in M58).  Usage is small, similar to "Recent Tabs" button.