Dark Mode: implement for paper-button

Spec/mocks: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit#slide=id.g471d243f08_0_874
Screenshots: https://imgur.com/a/6oYJZJF

BUG=883049
R=aee@chromium.org

Change-Id: I336867bdc95dc439ed43ee8c5782df5b6bd6e1f7
Reviewed-on: https://chromium-review.googlesource.com/c/1402347
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621699}
2 files changed