[w3c/FileAPI] Rename EndingTypes to EndingType or EndingsType (#91)

Sorry, missed this during the review.

The `EndingTypes` enum is named with a plural which doesn't make sense when used in the dictionary:
```webidl
dictionary BlobPropertyBag {
  DOMString type = "";
  EndingTypes endings = "transparent";
};
```
There's only one _type_ so the property type shouldn't be _types_. 

Either `EndingType` or `EndingsType` makes more sense.


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