Fix additionalMarginStart for inline-blocks

DRQS 39597130
RenderBlocks that are inline would have siblings that are not boxes.  This was
causing a crash when we tried casting them to RenderBox.  We'll just return 0
for inline-blocks.
Fix additionalMarginStart for inline siblings

DRQS 40112064
Return 0 if our parent has inline children, because this means we don't have
sibling boxes.  This has wider scope than the isInline() check that was added
earlier.  We may be able to remove that isInline() check later when we clean
this up.