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We also need to sort and combine the histogram-suffixes-list node in the
merge_xml. The pretty-print function assumed there is only one
histogram-suffixes-list node and it dropped all histogram-suffixes nodes
in the split histogram_suffixes_list.xml file which resulted in missing
all histograms that were generated from that xml. Added unit tests to
make sure this problem won't happen again.

This cl also renames histogram_suffixes.xml to
histogram_suffixes_list.xml to be consistent with histograms.xml naming
convention.

Bug: b:164097807
Change-Id: I67144c5726acd5aa96ce1a1f597c7bf88e7eda6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2375936
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801615}