also enable WebGL in TorButton

WebGL has been enable in TorBrowser in #6370 and is managed by NoScript.

But TorButton is still disabling WebGL at first start.

WebGL can still be re-enable in "about:config", but it was already like this before #6370 (closed).

Shouldn't TorButton allow WebGL now that it is only used in TorBrowser ?

Trac:
Username: cced