[LayoutNG] Add NGPhysicalLineBoxFragment::IsEmptyLineBox()

This patch adds IsEmptyLineBox() function that returns
whethe the line box is "empty", or "certain zero-height
line box" as defined in CSS2[1] or not, and change where
we used to determine it by checking if the number of children
is 0.

[1] https://drafts.csswg.org/css2/visuren.html#phantom-line-box

Bug: 636993, 903578
Change-Id: I2b9961e1ecb743063238b39a3a5be53f15ffce8c
Reviewed-on: https://chromium-review.googlesource.com/c/1332972
Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608419}
9 files changed