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In this CL, ScrollArrowView inherits the ShelfButton. It improves the
code readability and also makes Shelf code coherent: in current code
base all of buttons in Shelf are ShelfButtons.

Bug: 973481
Change-Id: I73aab558948b4ef32f79fa6e4ce243e9eba269a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772293Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690918}