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To solve the issue 1012772, we need to implement an interface to
scroll the shelf view to show the icon for a given app id. This CL
provides the following helper functions:
(1) Calculates the offset distance of scrolling Shelf to the
previous/next page.
(2) Calculates the layout strategy for the given scroll distance.
(3) Calculates the first/last tappable app indices for the given
layout strategy and scroll offset.

No UI changes should be introduced by this CL.

Bug: 1012772
Change-Id: I3aa27d0fa035f9a9a3186bbe9963065afe465f89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762558Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715870}