Allow user *not* to download Widevine

[Nathan Teodosio]

As all software in about:components, Widevine is automatically downloaded (and updated) by the browser, being installed locally to the Chromium snap under $HOME/snap/chromium/common/chromium/WidevineCdm.

Due to the nature of Widevine, namely non-free software to access DRM-gated media, users should be able not to have it installed and thus possibly arbitrarily loaded, especially if they do not actually ever access such media.

A --disable-component-update flag exists, but it disables all components. Given that some of them are really useful, for example 'safety tips', that presents dissuasive warnings when trying to access pages with malware[2], it is not a great solution.

A viable solution is to implement a flag to disable just Widevine.

[2] https://testsafebrowsing.appspot.com/