Add script to migrate suffixes to variants

Add suffixes_to_varaints.py to support migrating all histogram_suffixes
to new patterned histograms.

The script currently only handles the simplest case, i.e. Suffixes with
only one affected histogram to migrate to inline histograms.

Bug: 758782
Change-Id: I32905df5df5d00310c3029674a95d2705dab2f7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464207Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816853}

tools/metrics/histograms/pretty_print.py

@@ -15,13 +15,10 @@ and wrapping text nodes, so we implement our own full custom XML pretty-printer.
 from __future__ import with_statement
 
 import argparse
-import logging
 import os
-import shutil
 import sys
 
 sys.path.append(os.path.join(os.path.dirname(__file__), '..', 'common'))
-import diff_util
 import etree_util
 import presubmit_util