This patch fixes safe-to-break offset vector in ShapeResult:
1. Fixed before the first glyph of each run to be safe-to-break.
2. Fixed before the last glyph of each run not to force safe-to-break.
3. Fixed non-cluster boundaries were off by one.

Also related, two fixes are made to NGLineBreaker:
1. When the whole item can fit, it does not consider when the start of
   the item is not safe-to-break. The optimized code path was removed
   to fix this. If this seems to hit performance, we can bring it back
   with safe-to-break support.
2. Add |start_should_be_safe| argument to ShapingLineBreaker to make
   the start of wrapped line safe-to-break, but not the middle of a
   wrapped line (i.e., when there are previous items.)

HarfBuzzShaperTest.SafeToBreakLatinDiscretionaryLigatures was fixed
to match to what HarfBuzz returns as HB_GLYPH_FLAG_UNSAFE_TO_BREAK.

This change causes a crash in
fast/inline/absolute-positioned-inline-in-centred-block.html
This is from an issue in HarfBuzzShaper, tracked in issue 817271.

Bug: 816614
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Id55ea10f0b6f879981e3725b7cac027b39f3f213
Reviewed-on: https://chromium-review.googlesource.com/939942
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539839}