Add UI>Shell>Shelf component to c/b/ui/ash/launcher/OWNERS

Bug: none
Change-Id: I5faa2d53ffaf353d6209cfc68b5b0f83d6081f90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631941
Auto-Submit: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663926}

chrome/browser/ui/ash/launcher/OWNERS

 skuhne@chromium.org
 stevenjb@chromium.org
 khmel@chromium.org
+
+# COMPONENTS: UI>Shell>Shelf
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