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Function to split histograms by prefix

- Amended the histogram_paths.py to prepare for the distributed
directories of histograms after splitting.
- Amended two other files that previously expected a single
histogram.xml file.
- Added sorting of histogram nodes by name in merge_xml.py.

Change-Id: I60101777806584722cbeb778c778553259419122
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337155
Commit-Queue: Wenhan (Han) Zhang <zwenhan@google.com>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796655}