make sure gUM demos work in latest Firefox

[miketaylr]

https://blog.mozilla.org/futurereleases/2013/01/12/capture-local-camera-and-microphone-streams-with-getusermedia-now-enabled-in-firefox/

right now uses moz prefix and needs to be enabled via about:config.

[tagawa]

For reference, the flag in FF about:config is media.peerconnection.enabled

Also, some things to be aware of, in addition to prefixing getUserMedia:

• FF gUM doesn't like the autoplay attribute. Use video.play() in the script instead.
• window.URL.createObjectURL does exist but FF uses stream like Presto.
• FF uses video.mozSrcObject instead of video.src
• There are no video.videoWidth or video.videoHeight properties (and they're now set to zero in Chrome, it seems).

UPDATE: video.videoWidth and video.videoHeight properties do exist but only have a non-zero value once the "playing" event (and not the "play" event) has fired.

[tagawa]

More observations:

• FF fires the "playing" event repeatedly so if adding a listener for it, it should probably be removed later.
• FF requires the third getUserMedia() argument (error callback).

[tagawa]

Closing, if everyone's happy with the merged edits.

[miketaylr]

Awesome, thanks @tagawa !