Define to remove selection range automatically if a selection range is modified into different selection root

[masayuki-nakano]

Related to whatwg/dom#772

Currently, Chrome and Firefox returns mutable reference of Range object from Selection.getRangeAt(). Then, the Range can be updated outside the DOM tree, e.g., into a different document, into a disconnected tree. I wrote a test, then, both Chrome and Firefox get same result.

I believe that Selection should not keep storing a range whose boundary points' root is not the document associated with the Selection because this kind of checks are done at Selection.addRange() at least.

Cc: @sefeng211, @smaug----, @zcorpan

[dizhang168]

This might be a duplicate of: #79