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This adds the UI only. This is not yet functional, because the link to
native is not yet implemented. I plan to submit the necessary native CLs
first and then do the required JNI plumbing to stitch everything
together.

Keeping things separate makes for easier developing and (hopefully)
reviewing. See screenshot of the UI in the attached bug.

Bug: b/171776026
Change-Id: Ic7da1904a6a3c33d19c780c7dd5b9450599ba443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510053
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#823585}