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Splits histograms.xml into smaller files by prefix so resulting files
can be indexed and searched. Splits at given depth and aggregate
threshold.

Change-Id: Ia368af468c8229428b7079ce3a32ecd7d69f4ff1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2327990Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Wenhan (Han) Zhang <zwenhan@google.com>
Cr-Commit-Position: refs/heads/master@{#795693}