[cssom-view] Element.scrollIntoView should just have the "top" argument optional

Instead of having the current IDL and then prose that says what to do 
when called with 0 args, why not just do:

   void scrollIntoView(optional boolean top = true, optional 
ScrollOptions options);

?

Unless the intent is to have calls like this:

   scrollIntoView(undefined);

be equivalent to scrollIntoView(false), of course...

-Boris