You can finally say goodbye to that desktop Music Manager app for Google Play Music. Well, as long as you don't mind venturing into the Play Music labs. Google has added a new Chrome app toggle in the labs that enables drag-and-drop music uploads and a cool little pop-out player interface.
Just head to the labs page and enable "Google Play Music for Chrome" and save your changes. Chrome will download the extension, and then you can drag any compatible song files into the Play Music window to upload. There's also a button in the upper right to pull up a few more options for adding tunes in the browser. This should also mark the first time Chromebook users can add songs to Google Play Music, finally.
In the lower right is the new pop-out player toggle, which is part of the Chrome extension. This plugs into your main Play Music window (which has to remain open) to let you control playback and activate I'm Feeling Lucky Radio. All it's really missing is playlist control. I'm not seeing any stability issues on my end, but it's still technically a labs experiment, so be vigilant.