How to install Widevine CDM with Chromium on macOS?
I was able to successfully install Widevine by adding the following arguments to my build config:

ffmpeg_branding="Chrome"
proprietary_codecs=true
enable_widevine=true


First and second one makes sure that the codecs are available to play most video. enable_widevine well enables widevine.

Once built, go to chrome://components and scroll to the "Widevine" one. It should list the version as "0.0.0.0" or something like that. Click "Update". This should change the numbers to the most current version and install it. Restart the browser and it should work.