Update gitignore after splitting histograms

Add /tools/metrics/histograms/histograms_xml/*.before.pretty-print.xml
to gitignore to ignore generated before.pretty-print files for split
histograms as well as histograms_xml/obsolete_histograms.xml and
histograms_xml/histogram_suffixes_list.xml

Change-Id: I308bdfe8ff205def461418aca7a527c4ca3998a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437729
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814717}

.gitignore

@@ -298,6 +298,7 @@ vs-chromium-project.txt
 /tools/json_schema_compiler/test/json_schema_compiler_tests.xml
 /tools/metrics/actions/actions.old.xml
 /tools/metrics/histograms/histograms.before.pretty-print.xml
+/tools/metrics/histograms/histograms_xml/*.before.pretty-print.xml
 /tools/metrics/histograms/enums.before.pretty-print.xml
 /tools/page_cycler/acid3
 /tools/skia_goldctl/