Yeah. You say that retroarch is "Cross-Platform" yet chrome OS users are left squandering in their boring pieces of plastic.

As far as I know, Chrome OS is capable of running Android apps.

Retroarch has support for Android devices, so you should be able to install the Android version on your Chromebook. You can also tweak the settings of the Android version to make it look like the desktop version.

If that doesn't meet your standards, Retroarch also has support for web browsers (see official website).

I don't own a Chromebook myself, but I believe these options should do it.

Check this reddit thread covering the same issue for more info on the topic.