[git cl format] Actually format metrics XML files (fix an indentation error)

I introduced an indentation error in [1] which caused "git cl format" to
typically do nothing for histograms.xml and other metrics XML files. Oops.

[1] https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/569105

R=dpranke@chromium.org

Bug: 757939
Change-Id: Iab9975588156fca29e119e889ae634c5f77cb704
Reviewed-on: https://chromium-review.googlesource.com/627590Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>