This is nothing like XUL. It's just some abstraction layers written in C++. The hygiene from the Chromium side is that it allows us to share some code with abstractions we already have for the web platform.

Chrome UI on Windows, ChromeOS and soon Linux are comprised of a few layers:

Views (widget toolkit abstraction) - tabs, buttons, textfields, etc. Aura (window/"native widget" abstraction, event dispatching, what most code at layers above would consider to be the XWindow, except the XWindow is abstracted (and there is only one XWindow per top level browser window)) CC - (Chrome Compositor, also used for web content.)

They're all in native code since being able to drop down to the metal (& grab the app's toplevel XWindow) is often useful when building a desktop UI. HTML/JS/CSS binding to CC is limited to Blink.