GitLab

Adds the "AppListMixer" Finch experiment, affecting the search result
ranking and mixing in the App Launcher. By default, the mixer behaves
exactly as before. With the "Blended" option, results are selected and
ordered more naturally according to their relevance to the query, rather
than in arbitrary groups ("apps, then omnibox, then webstore, then
people").

Specific changes under the "Blended" option:

- Replaced the per-group "boost" with a per-group "multiplier". Now,
  instead of stratifying results from each group, they are mixed
  together according to the relevance score assigned by the provider.
  Some groups have a multiplier to de-prioritize them (people results
  x0.85, webstore results x0.4). This means that relevance scores now
  need to be comparable between groups!

- Removed the 6-result limit from the mixer. It can now return an
  arbitrary number of results, which will later be truncated by the
  view. (This is necessary because the view now supports more than 6
  results, since all app results take up a single row.)

- The per-group limit is now a "soft" limit. If there aren't enough
  total results to fill a minimum quota of 6, we go back and get as many
  results as we can from each group.

- The omnibox group is no longer special. Previously, it had no limit,
  but would only provide more than 1 result if there was room. Now, it
  has a soft limit of 4, and behaves the same as other groups (can
  provide more results if necessary).

Can be enabled with --force-fieldtrials=AppListMixer/Blended.

BUG=487494,460451,422610,338418

Review URL: https://codereview.chromium.org/1113483002

Cr-Commit-Position: refs/heads/master@{#329601}