Introduce a new cube sample that utilizes the feature level 0 (FL0) on
the web. The `Engine.create` method accepts `config` as an argument,
allowing users to explicitly request a GLES 2.0 context.
In addition, this change helps a future implementation of asynchronous
support for web, allowing the asynchronous mode to be turned on via the
configuration setting.
BUGS=[476134614]