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|last_visible_index_| of the main shelf view is updated in
CalculateAppCenteringStrategy. However, when the scrollable shelf
is enabled, |last_visible_index_| may be accessed before that function
is called. As a result, an illegal index value may be used. Given
that in scrollable shelf, |last_visible_index_| is always the index
of the last shelf item. So updates |last_visible_index_| when
a shelf item is added/removed.

Bug: 1006165
Change-Id: I4bc13e204835828bb1d5f77024ce274b9899ced0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816142Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699020}