These two are only used in contenteditable for visual left/right caret movements.

Hence P3.

Can we just disable the tests for LayoutNG in the meantime then?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/00b0d29817ee895c4cb45c32fc80f719a203593e

commit 00b0d29817ee895c4cb45c32fc80f719a203593e
Author: Xiaocheng Hu <xiaochengh@chromium.org>
Date: Fri Jan 18 20:55:27 2019

[LayoutNG] Suppress SelectionModifierTest failures

LeftPositionsOf() and RightPositionsOf() are for visual caret movement
in editable text only. Suppress them for now since we are still falling
back to legacy for contenteditable.

Bug:  922409 
Change-Id: Ic14a399aad93079266e031b1f58cd00d1a0c8678
Reviewed-on: https://chromium-review.googlesource.com/c/1422843
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624276}
[modify] https://crrev.com/00b0d29817ee895c4cb45c32fc80f719a203593e/third_party/blink/renderer/core/editing/selection_modifier_test.cc