Cherry pick HarfBuzz reverse_range fix

Cherry pick the HarfBuzz commit 8ac345e5c0ed0aad6547592ea0839aabfb4ba980
as it fixes a set of serious shaping regressions in the core text shaper
on Mac OS. Cherry picking as opposed to rolling to allow for a clean DI.

BUG=459043
TBR=behdad@chromium.org

Review URL: https://codereview.chromium.org/971153002

Cr-Commit-Position: refs/heads/master@{#318901}

third_party/harfbuzz-ng/src/hb-buffer.cc

@@ -454,7 +454,7 @@ hb_buffer_t::reverse_range (unsigned int start,
     info[j] = t;
   }
 
-  if (pos) {
+  if (have_positions) {
     for (i = start, j = end - 1; i < j; i++, j--) {
       hb_glyph_position_t t;