App list search results mixer: Fix invalid DCHECK and rewrote algorithm.
Rewrote the algorithm for adding omnibox results so it can deal with
having more than 6 non-omnibox results. Previously, it would
DCHECK-fail, and on release it would add all of the omnibox results. Now
it removes people and webstore results until there is one slot free for
the omnibox result.
Added some test cases that have people results.
BUG=432763
Review URL: https://codereview.chromium.org/
718353002
Cr-Commit-Position: refs/heads/master@{#304167}