[webkit-gtk] Encrypted Media support

[Philippe Normand]

Hi,

I'd like to add some EME support in WebKitGTK+. Here's what I thought
about:

- disable EME in default production build
- downstream users would need to enable it if they need it
- add a GStreamer ClearKey decryptor in platform/graphics, that would
depend on GStreamer >= 1.5
- other DRM systems would need to be handled by downstream users

The reason why this would depend on very recent GStreamer versions is
that DRM support in upstream GStreamer was recently added.

Any thoughts about this proposed plan?

Philippe