We record last_visit_dates separately for mobile and for desktop.

Martin:
Currently when clearing data in Chrome on any platform, we clear both the entries. Should we do so or is it enough
 - to clear desktop data when clearing on desktop and
 - to clear mobile data when clearing on mobile?

Martin is OOO. I suggest to delete the data for both platforms as we also delete the browsing history for both platforms.

+ tschumann

Let's aim for M-57. Since this isn't a user visible change, this can also land shortly after FF.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/26e30ced5c0018ccada8063ee8b0bcd2fac293af

commit 26e30ced5c0018ccada8063ee8b0bcd2fac293af
Author: tschumann <tschumann@chromium.org>
Date: Tue Jan 10 00:36:47 2017

Respect time range in browsing data removal for last-visited data.

This CL changes the data removal logic in last-visited data to only remove metadata
of nodes with a last-visited data inside the provided time-range.
Previously, that metdata was deleted from all nodes. This had the result that no
bookmarks were present in content suggestions unless they had been visited again.

BUG= 674178 

Review-Url: https://codereview.chromium.org/2616633002
Cr-Commit-Position: refs/heads/master@{#442424}

[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/chrome/browser/browsing_data/browsing_data_remover_unittest.cc
[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/chrome/browser/browsing_data/chrome_browsing_data_remover_delegate.cc
[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/chrome/browser/browsing_data/chrome_browsing_data_remover_delegate.h
[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/components/ntp_snippets/bookmarks/bookmark_last_visit_utils.cc
[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/components/ntp_snippets/bookmarks/bookmark_last_visit_utils.h
[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/components/ntp_snippets/bookmarks/bookmark_last_visit_utils_unittest.cc
[modify] https://crrev.com/26e30ced5c0018ccada8063ee8b0bcd2fac293af/components/ntp_snippets/bookmarks/bookmark_suggestions_provider.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/25e7324ca818226f5f68aca6eae4348ae739b247

commit 25e7324ca818226f5f68aca6eae4348ae739b247
Author: tschumann <tschumann@chromium.org>
Date: Thu Jan 12 16:46:42 2017

Add a unit test for the BookmarkSuggestionsProvider.

This is primarily to verify behavior fixed in https://codereview.chromium.org/2616633002/ but also adds tests for more fundamental behavior of the BookmarkSuggestionsProvider.

This CL also moves ContentSuggestionsProvider test doubles out of specific unit tests and into separate targets so that we can reuse them.

BUG= 674178 

Review-Url: https://codereview.chromium.org/2618243004
Cr-Commit-Position: refs/heads/master@{#443260}

[modify] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/BUILD.gn
[modify] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/bookmarks/bookmark_suggestions_provider.cc
[add] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/bookmarks/bookmark_suggestions_provider_unittest.cc
[modify] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/content_suggestions_service_unittest.cc
[add] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/fake_content_suggestions_provider_observer.cc
[add] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/fake_content_suggestions_provider_observer.h
[add] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/mock_content_suggestions_provider.cc
[add] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/mock_content_suggestions_provider.h
[modify] https://crrev.com/25e7324ca818226f5f68aca6eae4348ae739b247/components/ntp_snippets/remote/remote_suggestions_provider_impl_unittest.cc

+jkrcal, I believe this is fixed now. Marking as such -- please re-open if you disagree.