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When ScrollableShelf is enabled, the shelf icon under drag is actually
the proxy icon created by ScrollableShelfView while the original shelf
icon is transparent. However, when dragging an icon which is already
pinned on Shelf from AppsGridView, if the drag gets canceled, the
original shelf icon fails to show. This CL fixes such an issue. A
unit test is created for this scenario.

Bug: 1021768
Change-Id: Ibfce9ae3c59c21ac7d7e515b3a3a733e453fe5b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902511Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713630}