fix #1374 utils.Dom.calculatePosition|getOffset discrepancy in webkit

Due to discrepancy how browsers report page scroll (setting `scrollTop`
either on `document.body` or `document.documentElement`),
`aria.utils.Dom.getOffset()` returned wrong values in webkit browsers
for absolutely positioned elements when the viewport was scrolled.

[see https://code.google.com/p/chromium/issues/detail?id=284141]

The exact point of failure was querying explicitly
`calculatePosition(document.body)` which happens when `getOffset(element)`
traverses element's offsetParents up to BODY.

Close #1375