The HTML5 client does not have a layout drop down menu. It no longer uses windows and instead makes use of panels that can be shown/hidden by the user.
To enlarge the presentation area, the users can select the user list toggle in the upper left hand corner. This will minimize or hide the users list and messaging panels, giving preference to the presentation.
When webcams are being shared, the user can swap the presentation area for the webcams to create a webcam meeting (this option is not available when no webcams are shared):