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The previous CL was reverted because it assumes that the title of the
first shelf icon is "Chromium", which is not true under official
build. This CL replaces the hard-code check with the function to
fetch the icon's title. Meanwhile, this CL merges shelf_widget_test_api
into shelf_test_api.

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Previous CL description:

This CL accomplishes the following things:

(1) Designate the previous/next focusable widget for navigation widget,
hotseat widget and status area widget. It prevents the accessibility
ring from being trapped in one widget.

(2) Ensures that the hotseat is shown when the shelf app icon gets
the accessibility focus.

(3) Implements the test api to facilitate browser tests to access
the shelf widget code.

Bug: 1010219
Change-Id: Ibe522a6396c61f8b9bb7af01f34106f21500fdf6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1892155
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711490}