[FileAPI] Use `optional` for the Blob constructor (#33)

Currently, the spec has (boring parts omitted):
```
[Constructor, Constructor(sequence<...> blobParts, optional BlobPropertyBag options)] 
interface Blob {
```
It would probably be better to use
```
[Constructor(optional sequence<...> blobParts, optional BlobPropertyBag options)] 
interface Blob {
```

Note that this handles
```js
new Blob(undefined)
```
differently: the current spec throws a `TypeError`; after the change, it would be equivalent to `new Blob()`.

CC @bzbarsky 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/FileAPI/issues/33