[LayoutNG] Fix multi-glyph-cluster case for PositionData

This patch fixes ShapeResult::ComputePositionData,
CachedPositionForOffset, and CachedOffsetForPosition for when
multiple glyphs have a same character (cluster) index.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I862117d1ac19d7950306edc574be8f3e9b03ccc3
Reviewed-on: https://chromium-review.googlesource.com/1153068
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578871}
5 files changed