Bug 1813303 - Tweak order of operations in AppWindow::BeforeStartLayout. r=smaug

Sync the chromemargin attribute to the widget before restoring the
sizes, since the later depend on the former.

This is the same order we use when sizing the shell in OnChromeLoaded,
so it should be more consistent.

Differential Revision: https://phabricator.services.mozilla.com/D168200

xpfe/appshell/AppWindow.cpp

@@ -2556,8 +2556,11 @@ void AppWindow::ApplyChromeFlags() {
 NS_IMETHODIMP
 AppWindow::BeforeStartLayout() {
   ApplyChromeFlags();
-  LoadPersistentWindowState();
+  // Ordering here is important, loading width/height values in
+  // LoadPersistentWindowState() depends on the chromemargin attribute (since
+  // we need to translate outer to inner sizes).
   SyncAttributesToWidget();
+  LoadPersistentWindowState();
   if (mWindow) {
     SizeShell();
   }