Log Message:
Updated harfbuzz to 1.5.1.

Overview of changes leading to 1.5.1
Tuesday, September 5, 2017
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- Fix "unsafe-to-break" in fallback shaping and other corner cases.
  All our tests pass with --verify now, meaning unsafe-to-break API
  works as expected.
- Add --unicodes to hb-view / hb-shape.
- [indic] Treat Consonant_With_Stacker as consonant.  This will need
  further tweaking.
- hb_buffer_diff() tweaks.