GitLab

 - Extra methods added to RecurrenceRanker and RecurrencePredictor to accept queries.

 - Storage of targets has been moved from the RecurrenceRanker to individual
   RecurrencePredictors. As a result, RecurrencePredictor must now handle targets
   renames and removes themselves. Methods for this have been added to their API.

   This is necessary because query-based predictions will require a different data
   structure to store target + query than zero-state predictions, which only require
   a target. In order to keep the RecurrenceRanker usable for both tasks, choice of
   data structure needs to moved into the RecurrencePredictors, where individual
   predictors can use what's suitable.

 - The original FrecencyPredictor has been renamed ZeroStateFrecencyPredictor.

 - Some extra tests added.

Bug: 921444
Change-Id: I95a6dca135928726c779261ec6b1663b1023bf1f
Reviewed-on: https://chromium-review.googlesource.com/c/1459856Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630682}