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When the scrollable shelf is enabled, use ScrollableShelfView as the
shelf button delegate for the shelf app buttons. It helps to build
the connection between the app button and the topmost view in hotseat
widget. This CL should not bring any noticeable change.

Bug: 1010219
Change-Id: I0e4e3a810a72094a108af459561ebb306808c2f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869554
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709268}