GitLab

This patch adapts accessibility module to LayoutNG by:
 * Make |AbstractInlineTextBox| as an interface like class and implements legacy
 version |LegacyAbstractInlineTextBox| using InlineTextBox and LayoutNG
 version |NGAbstractInlineTextBox| using NGPaintFragment.
 * Make |AXLayoutObject| to handle |LayoutNGListMarker|
   - |LayoutListMarker| has no children.
   - |LayoutNGListMarker| is a container of list marker.
 * Make |AXLayoutObject::ComputeAccessibilityIsIgnored()| to handle LayoutNG
 block flow to check empty block flow.
 * Make |NGPaintFragment::FragmentRange::iterator| to work with |std::next()|
 * Intrdouce LayoutText::GetFirstLetterPart() for ease of getting layout object
  for first-letter part from layout object for remaining part.

Following patch will implements NGAbstractInlineTextBox::CharacterWidths()

The doc,https://bit.ly/2kUEFZy, contains study and change details.

TBR=dmazzoni@chromium.org

Bug: 714962
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iae25e8a7d7e6be0c7478470c690e081084476e85
Reviewed-on: https://chromium-review.googlesource.com/1104082
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569235}