Add support for mid ligature positions

Makes XPositionForOffset and CharacterIndexForPosition support
return results inside a ligature glyph (while still not breaking ZWJs)
by using a character-based TextBreakIterator inside glyphs.

Bug: 473476
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: I723cc8ba081958ff2d470d3e73ead6cbe327cf60
Reviewed-on: https://chromium-review.googlesource.com/1085554
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575496}
86 files changed