[LayoutNG] Fix empty inlines to influence the used line height

This patch matches NGInlineLayoutAlgorithm to CSS2 10.8 Line height
calculations: the 'line-height' and 'vertical-align' properties[1]
defining empty inline elements should influence line height.

Also, since the introduction of NGInlineBoxState, we had two places to
compute the union of boxes; NGInlineBoxState.stack_[0] and
NGLineBoxFragmentBuilder. Some code were uniting boxes to wrong one.
This patch unifies them to NGInlineBoxState.

[1] https://drafts.csswg.org/css2/visudet.html#line-height

BUG=636993

Review-Url: https://codereview.chromium.org/2845493002
Cr-Commit-Position: refs/heads/master@{#467278}
6 files changed