Make HTMLImports polyfill more robust when assigning baseURI property

This came up within the context of Cordova apps (https://github.com/MobileChromeApps/mobile-chrome-apps/issues/450)

On iOS, doc.baseURI is null, and the assignment throws a `TypeError`:

    TypeError: Attempted to assign to readonly property.

I actually don't see when it wouldn't be null since this is a new document. On Chrome, it doesn't throw, but the assignment also doesn't stick.

Instead of calling the setter, this PR creates a new property on the object itself.