LibWeb+UI: Add internals API to set browser zoom

Author: https://github.com/InvalidUsernameException
Commit: 5cc9a5802d
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2982
Reviewed-by: https://github.com/AtkinsSJ

UI/Headless/HeadlessWebView.cpp

@@ -191,4 +191,10 @@ void HeadlessWebView::on_test_complete(TestCompletion completion)
     m_test_promise->resolve(move(completion));
 }
 
+void HeadlessWebView::update_zoom()
+{
+    client().async_set_device_pixels_per_css_pixel(m_client_state.page_index, m_device_pixel_ratio * m_zoom_level);
+    client().async_set_viewport_size(m_client_state.page_index, m_viewport_size);
+}
+
 }