Move the popular sites factory next to it.

Currently PopularSites is explicitly not enabled (not even instantiated)
on desktop. We could in theory create it and let the field trial decide
whether to show tiles from it, but it really needs some work first for a
world where we use thumbnails instead of icons. Also, Desktop has its
own, separate mechanism for displaying default tiles baked into
TopSites.

BUG=655622

Review-Url: https://codereview.chromium.org/2572533002
Cr-Commit-Position: refs/heads/master@{#438472}