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This file is no longer shared between OS and Browser Settings, and can
be fully migrated to JS modules.

Also, did minor cleanups now that were possible as a result of dropping
the Polymer2 codepath.

Bug: 1069258
Change-Id: Ia0da4ac10f2858fa4fc5fe460de3b96c203bc733
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343010
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796167}