GitLab

Adds the ability to specify a threshold replacement to apply when predicting using an Assist-Ranker model.

For Contextual Search the threshold is read from a
FieldTrial variations parameter associated with the existing
ContextualSearchRankerQuery Feature.

For Translate, threshold setting is supported but unused.

BUG=899134

Change-Id: Ia0b664ce1e0949f755e7d8898fb2769fc91ae536
Reviewed-on: https://chromium-review.googlesource.com/c/1312296
Commit-Queue: Donn Denman <donnd@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612324}