Enforce member validity via WebIDL

[ChunMinChang]

Fixes #252.
Fixes #303.

Let me know if I overlook something ;)

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Preview | Diff

[youennf]

If we go with those changes, should we use more WebIDL?
For instance:

• Make duration a required member of MediaPositionState.
• Add default values for position and playbackRate.
  It would also be good to have corresponding WPT tests that cover those changes.

[youennf]

I think the PR mostly looks good and should be completed with the following bits:

• Revert the negative to not positive check, 0 being allowed.
• Change the WebIDL to state that duration is unrestricted.
• Add a new check so that if duration is NaN, an exception is thrown.

@ChunMinChang, can you revise the PR according yesterday's discussion or do you want me to take over?

[marcoscaceres]

Isn't it possible to just default these in the IDL itself?

[marcoscaceres]

Can't it just be this?:

dictionary MediaPositionState {
  required unrestricted double duration;
  double playbackRate = 1.0;
  double position = 0.0;
};

That should set all the defaults and the duration being required.

[chrisn]

Conclusion from TPAC meeting: So we accept 0, positive, positive infinity but throw on negative and NaN.

[marcoscaceres]

Requesting changes based on the proposed WebIDL.

[youennf]

Let's continue the work at #304.
@ChunMinChang, thanks for your efforts and input.

[marcoscaceres]

Closing in favor of #304