Un-deprecate UserViewedSavingsSize histogram

This CL correctly deprecates the UserViewedSavingsPercent histogram for
which the code was removed already, and undeprecate the
DataReductionProxy.UserViewedSavingsSize histogram for which code still
exists.

Bug: 870886
Change-Id: I2fa8ab3dec5ad15b87b57afd9d6f9e1df8ba04b5
Reviewed-on: https://chromium-review.googlesource.com/1162822
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580998}