Roll HarfBuzz to 1.8.7

https://chromium.googlesource.com/external/github.com/harfbuzz/harfbuzz/+log/2b76767bf572364d3d647cdd139f2044a7ad06b2..b6fdcf4f8bd09e065c767939125861c9dc8ff18f

Tbr: drott@chromium.org
Change-Id: Ie65cb46846224bd90ffa86517a946c44f321f2c7
Reviewed-on: https://chromium-review.googlesource.com/1170170Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582401}

DEPS

@@ -161,7 +161,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling HarfBuzz
   # and whatever else without interference from each other.
-  'harfbuzz_revision': '2b76767bf572364d3d647cdd139f2044a7ad06b2',
+  'harfbuzz_revision': 'b6fdcf4f8bd09e065c767939125861c9dc8ff18f',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling catapult
   # and whatever else without interference from each other.

third_party/harfbuzz-ng/README.chromium

 Name: harfbuzz-ng
 Short Name: harfbuzz-ng
 URL: http://harfbuzz.org
-Version: 1.8.3
-Date: 20180711
-Revision: 2b76767bf572364d3d647cdd139f2044a7ad06b2
+Version: 1.8.7
+Date: 20180809
+Revision: b6fdcf4f8bd09e065c767939125861c9dc8ff18f
 Security Critical: yes
 License: MIT
 License File: src/COPYING