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

libcef/browser/views/window_view.h

@@ -14,7 +14,6 @@
 #include "libcef/browser/views/overlay_view_host.h"
 #include "libcef/browser/views/panel_view.h"
 
-#include "third_party/abseil-cpp/absl/types/optional.h"
 #include "third_party/skia/include/core/SkRegion.h"
 #include "ui/display/display.h"
 #include "ui/views/widget/widget_delegate.h"
@@ -120,9 +119,6 @@ class CefWindowView
 
   void MoveOverlaysIfNecessary();
 
-  absl::optional<float> GetTitlebarHeight(
-      const CefRefPtr<CefWindow>& window) const;
-
   // Not owned by this object.
   Delegate* window_delegate_;