Remove direct usage of InlineBox in Previous/NextLinePosition()

This patch removes all direct usages of InlineBox (other than
RootInlineBox) in Previous/NextLinePosition. This is in preparation
to adding LayoutNG version of Previous/NextLinePosition:

crrev.com/c/1618643

Bug: 877263
Change-Id: I1c6303d0ecd15388e180f83daf65bc2fafc25822
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618642
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661449}
3 files changed