This feature modernizes the Chromium scrollbars (both overlay and non-overlay) on Windows and Linux to fit the Windows 11 Fluent design language. Non-overlay Fluent scrollbars will be enabled by default in Linux and Windows. This change applies to Linux as well because Chromium's Linux scrollbar design has historically been aligned with what ships on Windows. How to expose enabling overlay Fluent scrollbars is still being decided. Visual spec: https://docs.google.com/document/d/1haDpb1QIh2PaLwsQD1i4WHFq_5_jSK3XK9lhgSs4WkM/edit?usp=sharing Design doc: https://docs.google.com/document/d/1GCmz2nbJV1XiopoLHnlrVaHCjhQMdiyDfPN_a22OIjU/edit?usp=sharing Intent to implement: https://groups.google.com/a/chromium.org/g/blink-dev/c/SomQdLx6aEk