Disable Widevine in Tor mode

[bsclifton]

Carried over from brave/browser-laptop#13626

[kjozwiak]

Still an issue, easiest way to reproduce, see the issue occurring:

• visit https://bitmovin.com/demos/drm

You'll notice that you'll get a Widevine notification and can install it inside a Tor Window. Example:

[iefremov]

Blocking the Widevine installation prompt is trivial, we have just to add a check for Tor profile in BraveDrmTabHelper::OnWidevineKeySystemAccessRequest. Do we also have to block playing Widevine videos in Tor windows if Widevine is already installed? @diracdeltas @bsclifton

[diracdeltas]

i think we should not allow widevine in tor mode even if already installed (same with flash if its installed) but defer to @tomlowenthal

[tildelowengrimm]

I agree with @diracdeltas on this one. My preference is to disable Widevine entirely in Tor: no prompt to install, and no playing if it's already installed.

[iefremov]

No playing goal should be achievable by disabling the content setting for plugins.