Enable Fullscreen API preferences by default

embedding/embedlite/embedding.js

@@ -302,3 +302,18 @@ pref("extensions.minCompatibleAppVersion", "11.0");
 
 // Enable sub layers for apzc
 pref("layout.want.subapzc", true);
+
+// Enable HTML fullscreen API in content.
+pref("full-screen-api.enabled", true);
+// But don't require approval when content enters fullscreen; we'll keep our
+// UI/chrome visible still, so there's no need to approve entering fullscreen.
+pref("full-screen-api.approval-required", false);
+// Don't allow fullscreen requests to percolate across content/chrome boundary,
+// so that our chrome/UI remains visible after content enters fullscreen.
+pref("full-screen-api.content-only", true);
+// Don't make top-level widgets fullscreen. This only applies when running in
+// "metrodesktop" mode, not when running in full metro mode. This prevents the
+// window from changing size when we go fullscreen; the content expands to fill
+// the window, the window size doesn't change. This pref has no effect when
+// running in actual Metro mode, as the widget will already be fullscreen then.
+pref("full-screen-api.ignore-widgets", true);