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Edge padding of scrollable shelf logically consists of two parts:
the minimum gap between scrollable shelf and other components
(such as status area widget) and the gap decided by centering strategy
(display centering, view centering or overflow). This CL separates the
edge padding into those two parts. It improves the code readability.

In addition, currently the shelf view does not notify the parent view
of the added icon. This CL also fixes it.

Change-Id: Id10263b7b532230539cac5d179d7534ca050ef87
Bug: 1041702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052593
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745522}