Bug 1816287 - Always exit fullscreen when triggering on external protocol r=edgar

It's a security risk such that the maximized external program can obscure
the fullscreen notification and the malicious site can use this trick
to load a spoofed page in the background without user notices it.

This patch minimized the risk by always exit the fullscreen mode when
an external protocol is triggered.

Differential Revision: https://phabricator.services.mozilla.com/D177771