Add YEAR_2021 tag

Bug: chromium:1158374
Change-Id: Idf64a3fc636f4250ad9bb3ea6008676ec9c2d905
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597437
Auto-Submit: Almothana Athamneh <almuthanna@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838145}

tools/perf/page_sets/system_health/story_tags.py

@@ -62,6 +62,7 @@ YEAR_2017 = Tag('2017', 'Story was created or updated in 2017.')
 YEAR_2018 = Tag('2018', 'Story was created or updated in 2018.')
 YEAR_2019 = Tag('2019', 'Story was created or updated in 2019.')
 YEAR_2020 = Tag('2020', 'Story was created or updated in 2020.')
+YEAR_2021 = Tag('2021', 'Story was created or updated in 2021.')
 
 
 def _ExtractAllTags():