Avoid partial rows of most visited icons on the NTP.

Avoid partial rows of most visited icons on the NTP.

This limits the number of rows of most visited tiles that will be
displayed on the new tab page, and hence ensures that we don't show
partial rows.

Previously on narrow devices, we'd show 8 tiles over three rows, which
feels unbalanced. Now we show just 2 rows (which happens to be 6 tiles).
In non-Google mode, we show 3 rows (up to 12 tiles).

Before:
 X X X
 X X X
 X X

After:
 X X X
 X X X

BUG=504367

Review URL: https://codereview.chromium.org/1275083003

Cr-Commit-Position: refs/heads/master@{#342946}