Explicitly convert raw data to strings

As strings are guaranteed to use UTF-8 in the GNOME platform, generic
file APIs like g_file_load_contents() return raw data instead. Since
gjs' recent update to mozjs60, this data is now returns as Uint8Array
which cannot simply be treated as string - its toString() method boils
down to arr.join(',') - so use gjs' new ByteArray module to explicitly
convert the data.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/179

js/ui/sessionMode.js

@@ -117,6 +117,8 @@ function _loadMode(file, info) {
     let fileContent, success, tag, newMode;
     try {
         [success, fileContent, tag] = file.load_contents(null);
+        if (fileContent instanceof Uint8Array)
+            fileContent = imports.byteArray.toString(fileContent);
         newMode = JSON.parse(fileContent);
     } catch(e) {
         return;