Abstract AXLayoutObject::CanIgnoreSpaceNextTo reading InlineTextBox

CL:988375 fixed AXLayoutObject::CanIgnoreSpaceNextTo by accessing
LayoutText::First/LastTextBox() directly. Since these functions
will be gone in LayoutNG, this patch abstracts what it does and
hide InlineTextBox inside of LayoutText.

Actual implementation of these functions for LayoutNG will be
done after CL:986982 adds retrieval of NGInlineItem. It's
possible with NGOffsetMapping or NGPhysicalFragment, but
NGInlineItem would be better fit.

Bug: 821906, 636993, 830546
Change-Id: If590e8c01e59d58b0f77ba6b3f57e35e9c29acb3
Reviewed-on: https://chromium-review.googlesource.com/997912
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549451}
4 files changed