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The {TabletOrClamshell} token is used in various different ash
histograms but they all use the same variants. This CL extracts the
variants into a common variants tag.

Bug: 1140182
Change-Id: Ifd9b5233405b65308c09b24e0e6d7700eef2db3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533362Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Jeremy Chinsen <chinsenj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826874}