MediaRecorder: update to spec (1/3)

Inspired by CL: https://crrev.com/1490403003/ ;)

- TreatUndefinedAs=missing is going to be removed from the
spec [1] and is already not present in Gecko implementation [2]

- Also, "BlobEvent.data is non-nullable, so the dict argument
should be required, and the data argument should be required
and non-nullable." [3]

- Removing suffix 'Enum' from RecordingStateEnum [4].

- BlobEvent has a constructor now [5], so remove the TODO.

BUG=262211

[1] https://github.com/w3c/mediacapture-record/issues/7
[2] https://github.com/mozilla/gecko-dev/blob/master/dom/webidl/MediaRecorder.webidl
[3] https://groups.google.com/a/chromium.org/d/msg/blink-dev/76HB0BIxk_o/9ZK0yXirBwAJ
[4] https://github.com/w3c/mediacapture-record/issues/8
[5] https://github.com/w3c/mediacapture-record/issues/11

Review URL: https://codereview.chromium.org/1497883002

Cr-Commit-Position: refs/heads/master@{#364518}
5 files changed