Move panel item CSS into component

Moves all the styles used by a panel item from the included
external CSS file into the custom element template.

Bug: 947388
Change-Id: I010f65de130cda1462cfe3714d63af59f916593e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1689394
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675432}
2 files changed