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Turns the swap mouse button toggle into a dropdown with Left button and
Right button as options, as per the bug.

String screenshots:
https://storage.cloud.google.com/chromium-translation-screenshots/0de32bdc0a9e5c95680629877c323c11d242cf03
https://storage.cloud.google.com/chromium-translation-screenshots/037c4cf7a5c655f6e15a4ca3c906230359440d50

Fixed: 1132518
Change-Id: I55836c82115a2fc1473d4863b2f96ed9b94f4a05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436750
Commit-Queue: Daniel Classon <dclasson@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813034}