Add a new guard ENABLE_MEDIA_PROCESS

[JulienIsorce]

For now media process is guarded under USE_GSTREAMER.
But Media Process has to be generic and should host any kind of media backend.

Adding this new guard also means to rename some classes we added. For example WebMediaPlayerGStreamer should be renamed to WebMediaPlayerProxy.
Also move this class to "content" namespace instead of "media".

Also move build in gst plugins into a new sub directory (rename gst_chromium_http_source to gst_chromium_http_src).