Update from https://crrev.com/316786

third_party/harfbuzz-ng/src/hb-ot-shape.cc

 /*
  * Copyright © 2009,2010  Red Hat, Inc.
  * Copyright © 2010,2011,2012  Google, Inc.
  *
  *  This is part of HarfBuzz, a text shaping library.
  *
  * Permission is hereby granted, without written agreement and without
  * license or royalty fees, to use, copy, modify, and distribute this
  * software and its documentation for any purpose, provided that the
  * above copyright notice and the following two paragraphs appear in
@@ skipping 658 matching lines @@
       if (space_status == SPACE_DONT_KNOW)
         space_status = c->font->get_glyph (' ', 0, &space) ? SPACE_AVAILABLE : SPACE_UNAVAILABLE;
 
       if (space_status == SPACE_AVAILABLE)
       {
         info[i].codepoint = space;
         pos[i].x_advance = 0;
         pos[i].y_advance = 0;
       }
       else
-        continue; /* Delete it. */
+        continue; /* Delete it. XXX Merge clusters? */
     }
     if (j != i)
     {
       info[j] = info[i];
       pos[j] = pos[i];
     }
     j++;
   }
   c->buffer->len = j;
 }
@@ skipping 104 matching lines @@
   hb_set_t copy;
   copy.init ();
   do {
     copy.set (glyphs);
     for (hb_codepoint_t lookup_index = -1; hb_set_next (&lookups, &lookup_index);)
       hb_ot_layout_lookup_substitute_closure (font->face, lookup_index, glyphs);
   } while (!copy.is_equal (glyphs));
 
   hb_shape_plan_destroy (shape_plan);
 }