Bug 1489385 part 2 - Make the refresh driver event dispatching specific to fullscreen events. r=smaug

The mechanism was added in bug 1168705 for fullscreen, and is only used
for this since then. The spec now has a specific step for this in the
event loop, so specialize this mechanism to match the spec closer.

This patch also changes the caller type of the fullscreenchange event
from non-system to system (via using DispatchTrustedEvent rather than
EventTarget::DispatchEvent(Event&)).

Differential Revision: https://phabricator.services.mozilla.com/D5235