This will allow git cl format to handle histograms.xml being
split into several files, and also adds support for UKM.

Bug:699328

Change-Id: I2bfbd333b78b679f69d7cb298f5adaf02f6a4dec
Reviewed-on: https://chromium-review.googlesource.com/483979Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>