views: mac: Support dynamic resize of title bar height (see #3189)

This is intended for usage with frameless windows that show the standard window
buttons, where resizing the title bar height changes the button offset. Returning a
different value from CefWindowDelegate::GetTitlebarHeight and forcing a resize of
the NSWindow's theme frame (see ViewsWindow::NudgeWindow) will update the
title bar height.

To test:
1. Run `cefclient --use-views --hide-frame --show-window-buttons --url=http://tests/window`
2. Enter a new value for title bar height and click the "Set Titlebar Height" button

tests/cefclient/browser/root_window_views.h

@@ -26,6 +26,8 @@ class RootWindowViews : public RootWindow,
   RootWindowViews();
   ~RootWindowViews();
 
+  void SetTitlebarHeight(const std::optional<float>& height);
+
   // RootWindow methods:
   void Init(RootWindow::Delegate* delegate,
             std::unique_ptr<RootWindowConfig> config,