Include Breeze scrollbar CSS inline

You cannot access cssRules for <link rel="stylesheet" ..> on a different domain since Chrome 64.
Since our chrome-extension:// URL for a stylesheet would be on a different domain than the website,
this can lead to problems in e.g. modernizr, so include the <style> inline instead.

Differential Revision: https://phabricator.kde.org/D17169