gfx/harfbuzz status as of 2017-12-20:

This directory contains the HarfBuzz source from the 'master' branch of
https://github.com/behdad/harfbuzz.

Current version: 1.7.4

UPDATING:

Note that gfx/harfbuzz/src/hb-version.h is not present in the upstream Git
repository. It is created at build time by the HarfBuzz build system;
but as we don't use that build system in mozilla, it is necessary to refresh
this file when updating HarfBuzz, and check it into the mozilla tree.

The normal approach to updating HarfBuzz, therefore, is to pull the latest HB
source into a scratch directory and do a local build; then copy the original
sources AND the generated header mentioned above from the build directory into
the mozilla tree.

If the collection of source files changes, manual updates to moz.build may be
needed, as we don't use the upstream makefiles.