The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9677e78a4a25747b9b4ea86b6536461125f0b235

commit 9677e78a4a25747b9b4ea86b6536461125f0b235
Author: mastiz <mastiz@chromium.org>
Date: Mon Dec 12 09:55:48 2016

ntp_tiles::metrics: Add rappor metrics for impression URLs per icon type.

The goal is to list the top domains that show non-real icons and then
debug them individually. If we find evidence that such domains do
actually provide large icons, it would suggest there's client-side bugs
that prevent from displaying this icon (either it wasn't fetched, the
wrong version was kept around, it was garbage-collected, etc.).

BUG= 672411 

Review-Url: https://codereview.chromium.org/2557513007
Cr-Commit-Position: refs/heads/master@{#437838}

[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPage.java
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/chrome/android/java/src/org/chromium/chrome/browser/profiles/MostVisitedSites.java
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/chrome/android/javatests/src/org/chromium/chrome/browser/ntp/FakeMostVisitedSites.java
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/chrome/browser/android/ntp/most_visited_sites_bridge.cc
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/chrome/browser/android/ntp/most_visited_sites_bridge.h
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/chrome/browser/ui/webui/ntp/ntp_user_data_logger.cc
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/components/ntp_tiles/BUILD.gn
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/components/ntp_tiles/DEPS
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/components/ntp_tiles/metrics.cc
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/components/ntp_tiles/metrics.h
[add] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/components/ntp_tiles/metrics_unittest.cc
[modify] https://crrev.com/9677e78a4a25747b9b4ea86b6536461125f0b235/tools/metrics/rappor/rappor.xml

Issue 235015 has been merged into this issue.

With the rollout of M57, first Rappor metrics are available, e.g.: https://uma.googleplex.com/rappor?day_count=90&metric=NTP.SuggestionsImpressions.IconsGray

Let's wait a few weeks to collect sufficient data.

Reopening to explore if UKM should be used instead of Rappor metrics.

Apparently, UKM doesn't fit well for this use-case, because the URL used as key is the main frame navigation URL.