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This change will allow users to use the mousewheel to scroll over
the contents of the scrollable shelf.

Bug: 1018016
Change-Id: I949eb241a74be365b0a366f56c59cba04ce6099b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901354Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Ana Salazar <anasalazar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713271}