Enable MHTML support.
Use "scripted" media controls.

* Instead of having to write custom C++ code for the media controls, we
can use a generic JS/CSS implementation.
* This shows the limits of our form drawing code, however, as the
buttons don't look good.
* Adapted from similar change for EFL:
https://github.com/WebKit/webkit/commit/8d9af4197aaef48aee538661c3c762f8575938c7