One problem caused by this: TopSites only considers navigations with transition TYPED or AUTO_BOOKMARK (see https://cs.chromium.org/chromium/src/components/history/core/browser/history_backend.cc?rcl=0&l=339).
This means that MostLikely clicks never fed into TopSites (maybe explaining the reports where ML users occasionally get really old suggestions: they might have fallen back to TopSites, which doesn't have any recent data to work with).

Even worse, after https://codereview.chromium.org/1908363002/ (M53), clicks on TopSites tiles themselves also don't feed into TopSites. That seems like it'd cause a significant quality regression.

Will take a look at these issues.

Marking as release blocker since we should really fix the TopSites regression. I see two options for that:
- Somehow make NTP tile clicks AUTO_BOOKMARK again (might be hard on desktop).
- Make TopSites also consider LINK transitions if the previous navigation entry is an NTP.

For the record, extracted treib@'s comment from https://bugs.chromium.org/p/chromium/issues/detail?id=619484#c21:

"Here's a summary of transition types produced by Chrome actions on Android (version 51.0.2704.81) 

Typing in URL in omnibox: TYPED 
Picking a suggestion: TYPED 
Opening link from gmail: LINK 
Opening an item in dropdown list in GSA: LINK 
Opening Most Likely item: AUTO_BOOKMARK 
Opening TopSites item: AUTO_BOOKMARK 
Opening regular bookmark: LINK 

Items of type LINK are effectively excluded from consideration in ML pipeline because the code requires it to follow another entry with same window id and tab id and NTP URL."

That was Igor's comment, not mine :)

Note that right now, MostLikely and TopSites are only AUTO_BOOKMARK on Android; on desktop they're both LINK.

On desktop (version 53.0.2763.0) regular bookmarks also appear as AUTO_BOOKMARK.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a77db699719e4ecb07ce3a37fe4698ae92372111

commit a77db699719e4ecb07ce3a37fe4698ae92372111
Author: mastiz <mastiz@chromium.org>
Date: Thu Jun 30 09:48:42 2016

Fix clicks on NTP tiles not contributing to Most Visited tiles

The code that computes the client-side list of Most Visited pages,
surfaced in the NTP, considers navigations of types TYPED or
AUTO_BOOKMARK (i.e. excludes LINK): see
https://cs.chromium.org/chromium/src/components/history/core/browser/history_backend.cc?l=339

The goal of this reinforcement is that a click on a tile increases the
likelihood of that page being listed the next time an NTP is opened.

This works fine for Android because clicks on NTP tiles produce
AUTO_BOOKMARK transitions.

However, for desktop, https://codereview.chromium.org/1908363002/ (M53)
caused a regression because it inadvertently changed the transition type
from AUTO_BOOKMARK to LINK (for TopSites tiles only, i.e. computed
locally). This means such clicks don't influence the ranking of tiles,
and therefore TYPED URLs dominate the list.

Note that the issue mostly affects non-signed-in users, since signed-in
users fetch a list from a remote service.

Manually tested:
Google remote NTP - transition type changed from LINK to AUTO_BOOKMARK.

BUG= 620296 
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2084953003
Cr-Commit-Position: refs/heads/master@{#403132}

[modify] https://crrev.com/a77db699719e4ecb07ce3a37fe4698ae92372111/chrome/browser/chrome_content_browser_client.cc
[modify] https://crrev.com/a77db699719e4ecb07ce3a37fe4698ae92372111/chrome/browser/chrome_content_browser_client.h
[modify] https://crrev.com/a77db699719e4ecb07ce3a37fe4698ae92372111/content/browser/frame_host/navigator_impl.cc
[modify] https://crrev.com/a77db699719e4ecb07ce3a37fe4698ae92372111/content/public/browser/content_browser_client.h

Marking this as fixed since it's release-blocker and the issue in #2 got fixed.