[IndexedDB] DOMStringList has been removed from DOM (#28)

Imported from https://www.w3.org/Bugs/Public/show_bug.cgi?id=28067

Relevant bits:

* Adding _Array.prototype.contains_ to ECMAScript is not Web-compatible; ES7 proposes _Array.prototoype.includes_ instead which is nice but not helpful here
* Web IDL has grown `FrozenArray<>` which is what newfangled specs would use
* Recent public-script-coord thread https://lists.w3.org/Archives/Public/public-script-coord/2015AprJun/0060.html suggests:
  * Let's gather metrics and see what we are forced to maintain - `contains`, `item`, both or neither - on each usage of _DOMStringList_
  * If necessary, let's add something to WebIDL like `FrozenArrayWithContains<>`

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/28