Offset: fix iframe scrollTop/Left test for IE8 and iPhone

* IE8 need a doctype, otherwise IE will scroll it, but will
  still show old values. It wasn't noticable before, since IE8 will
  update values if in the dev tools you swtich to "Quirks mode"
  and then back again, then this tab will always show the correct values
  even if you update it

* iPhone resize the iframe by its content regardless of
  the width, height values, meaning it's not possible to scroll
  the iframe only its parent element

Ref ae30fb6c27a2121578d3ce7ca6e2ba1d205545b8