Add quirk `elementBCRIgnoresBodyScroll` and it's
workaround in DomUtils.getBoundingClientRect.

When the body has scrolled, an element/range's BCR
should update to reflect the new coordinates.

Instead, in iOS Safari, an element's BCR turns out
to be different from the range's (correct) BCR,
because the element's does not take the body's scroll
into account.

This workaround attempts to trigger a quick bodyscroll
and detects this quirk by measuring the BCRs of a
test element vs it's range representation.