Explicitly cast from HarfBuzz type to shorter type

This allows the file to be compiled with C++11.

BUG=

Review URL: https://codereview.chromium.org/316113002

git-svn-id: svn://svn.chromium.org/blink/trunk@175606 bbb929c8-8fbe-4397-9dbb-9b2b20218538