Change OptimizationGuide histogram back to sparse histogram

Switch OptimizationGuide.PredictionModelUpdateVersion.* histogram back
to sparse histogram as we've implemented the SparseHistogram's WriteJson
related functions.

Bug: 990131
Change-Id: I7693fcde40b3f9d8b0c8101e883cb57c25c66656
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2240420
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Michael Crouse <mcrouse@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777411}