collapse and setBaseAndExtent methods contain redundant checks of offset

[darinadler]

In the specification of the collapse method there is a step that says that the method must throw IndexSizeError if offset is longer than the node’s length. This check is also done as part of creating newRange in subsequent steps. So the IndexSizeError exception will be thrown in either case. However, because the specification explicitly calls for doing this before checking the node’s root, this difference is observable in obscure cases.

I suggest we remove the explicit IndexSizeError step. Better for consistency with other operations in the Selection API that don't explicitly specify an IndexSizeError check. I think it’s likely this is not already needed for web compatibility, so it’s not too late to fix this.

These same considerations apply to the setBaseAndExtent method. And I suggest the same change, removing the explicit IndexSizeError step.

[darinadler]

The WPT tests for collapse and setPosition expect the IndexSizeError check to be done before the document.contains check. The WPT test for setBaseAndExtent is does not cover the case where there is both IndexSizeError and a document.contains violation and so does not detect if the check is done before or after the document.contains check.