[w3c/IndexedDB] Update to Web IDL changes to optional dictionary defaulting (#285)

See: https://github.com/heycam/webidl/issues/758

Web IDL recently changed to require explicitly listing a default value for defaulted dictionaries instead of it just magically happening.

In practice what that means is that APIs like this:
```webidl
Constructor (BaseAudioContext context, optional AnalyserOptions options)
```
need to be written like this instead:
```webidl
Constructor (BaseAudioContext context, optional AnalyserOptions options = {})
```
(with the addition of `= {}` on the dictionary argument). 

For Indexed DB:

* IDBDatabase (`createObjectStore()`)
* IDBObjectStore (`createIndex()`)
* IDBVersionChangeEvent (constructor)

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