The Web Browser widget is supported on desktop platforms as well as Android and iOS mobile devices.
Chromium is used for the web browser widget on desktop platforms while both iOS and Android use native WebKit controls aligned to match your UMG layout. This keeps executable size to a minimum and increases performance.
The Web Browser widget can be created through a Widget Blueprint or in C++ using Slate code and, as mentioned above, is currently considered experimental so the feature will need to be enabled from Edit menu under Plugins menu. Under Widgets, click Enabled on the Web Browser plugin to enable it.
Once enabled, you will find the widget in the Palette window on the Designer tab of a Widget Blueprint under Experimental.