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Previously on edge case screens, we use |LabelButton|, which is
unfocusable by default, for buttons without background. But these
buttons need to be focusable during tab traversal and accessibility
mode.

This CL introduces |InterstitialViewButton| as a subclass of
|RoundedLabelButton|. It handles both types of buttons (with and
without background), and it's focusable with a rounded rectangle focus
ring and shares common setup of the system menu buttons.

Screenshot when the button is on focus:
https://screenshot.googleplex.com/AKv42vcGDSsTYGV.png

BUG=1106937,1126208

Change-Id: I7511f8f10a0cb4abc07283d7fcee7ac79bacae31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450844Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814908}