Changed type of iframe to HTMLIFrameElement.
Changed type of iframe to HTMLIFrameElement.

Using a more specific type for `IChildConnectionObject.iframe` makes
working in TypeScript more pleasant; the compiler will no longer
complain about using properties specific to iframe elements (like `src`
or `name`) on that object.

Aaronius/penpal,Aaronius/penpal,Aaronius/penpal,Aaronius/penpal