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/window_test_runner.cc

@@ -36,5 +36,10 @@ void WindowTestRunner::ModifyBounds(const CefRect& display, CefRect& window) {
   }
 }
 
+void WindowTestRunner::SetTitleBarHeight(CefRefPtr<CefBrowser> browser,
+                                         const std::optional<float>& height) {
+  NOTIMPLEMENTED();
+}
+
 }  // namespace window_test
 }  // namespace client