It seems working with the latest chromium-freeworld almost out of the box.
Assuming a fresh installation with a clean profile, open chrome://components/, navigate to the section “Widevine Content Decryption” and click “Check for update”.
The “Version” should change to something like “4.10.1610.0” and the “Status” to “Component updated”.
Then restart the browser and try to pass the test: https://bitmovin.com/demos/drm