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Currently, the Switch Access back button is drawn by the extension, but
in order to match the visual specification (see bug) the back button
needs to be drawn using native views.

This change creates a bubble controller for Switch Access and adds the
ability to show the back button. This functionality is not yet used.

Relnotes: N/A
Bug: 973719
Change-Id: I6da3772e8183d0cdbd04b554a4764719d12b8e25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124234
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Katie Dektar <katie@chromium.org>
Reviewed-by: Abigail Klein <abigailbklein@google.com>
Cr-Commit-Position: refs/heads/master@{#756364}