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This CL enables the scrollable shelf to handle the mouse wheel event.

Bug: 1005002
Change-Id: I25914f3591f40e560a681cc41bedcfaa0ee35f68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822357Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699558}