Previously the widget document had `padding: 6px` on the body, so every
embedded widget sat 6px inside its container and could not align flush with
the host layout. `updateIframeHeight` then reported
`document.body.offsetHeight + 12`, but the body is `box-sizing: border-box`
and `offsetHeight` already includes padding, so the addition double-counted
it.
Measured against the deployed widget: the content needs 20610px, the body
reported 20622px with the padding, and the parent was told 20634px, leaving
24px of empty space below every embed on top of the horizontal inset.
Removing the padding does not clip anything. With it at zero, offsetHeight,
body scrollHeight and documentElement scrollHeight all agree, and the last
child carries no bottom margin, so no margin collapses through the body edge.
Resolves#1487.
Dmitry VerkhoturovandUmputun
fdfce6495c