Remove unnecessary "virtual"s in HistogramSamples

In preparation for some other CLs, so that they'll be easier to review.

Bug: 1293026
Change-Id: Ia724d703fe9e92f78449d291e158bdaf8d7f350d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4348353


Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118649}