Bug 1617750 - Lazy load hidden window for GeckoView. r=geckoview-reviewers,snorp

GeckoView doesn't need the hidden window loaded eagerly on startup. This
shaves off around ~100ms timing from XRE_main to DOMContentLoaded of
geckoview.xhtml.

Differential Revision: https://phabricator.services.mozilla.com/D63977

--HG--
extra : moz-landing-system : lando

mobile/android/app/geckoview-prefs.js

@@ -25,6 +25,8 @@ pref("dom.ipc.processPrelaunch.enabled", false);
 pref("toolkit.telemetry.isGeckoViewMode", true);
 // Disable the Telemetry Event Ping
 pref("toolkit.telemetry.eventping.enabled", false);
+// Don't create the hidden window during startup.
+pref("toolkit.lazyHiddenWindow", true);
 
 pref("geckoview.console.enabled", false);