Anomaly

Anomaly is a Chrome extension which scrobbles Google Music. There are plans to expand it to other web-based music sources such as YouTube, Hypem etc. The version which is currently available on the Chrome web store is specifically for Libre.fm. There is a "multi" version which though not released on the web store is also freely available and installable out of the box.

You can find it here:

https://gitorious.org/anomaly-for-gnu-fm