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Remove pretty-print and validate format comments in all xmls under
histograms_xml/. Update their comments as well.

Change-Id: Ifd3ceddb3e649bb3c28ffe6d8dc32b8a979a8b2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376670
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801845}