std-switch: Add basic style

Add basic style for 'off' and 'on' states.
This CL doesn't contain style for focus, active, hover and disabled states.

Bug: 972476
Change-Id: I0ce0e03788816b3dc4c388106d0325797a12fbac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674007
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672303}
7 files changed