[LayoutNG] Set kDontReshapeStart correctly when rewinding

NGLineBreaker::BreakText sets kDontReshapeStart for the items
at the beginning of a line.

This patch fixes to compute it by whether the start offset of
the item_result is at the beginning, not the current offset
NGLinebreaker is reading at. They are the same for the normal
line breaking, but not so when rewinding.

Found while investigating crbug.com/866771.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I812d2cd44492a0112bcd037749168e8eb62d8a94
Reviewed-on: https://chromium-review.googlesource.com/1190983
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586615}
1 file changed