This allows us to skip 'git cl format' for histograms.xml, which in
turn allows us to issue a presubmit error rather than a warning when
that file is malformatted, without running the formatting code twice.
The formatter is somewhat slow for this file, so it's helpful to avoid
duplicating that work.

BUG=none
TEST=none
R=iannucci@chromium.org

Change-Id: Ia86d45c7c56a9f89db34ca7175ef686589084f85
Reviewed-on: https://chromium-review.googlesource.com/569105
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>