Set USE(HARFBUZZ) in config.h instead of a gyp file.

USE(HARFBUZZ) does not need to be exposed to the gyp build system.

BUG=286578

Review URL: https://chromiumcodereview.appspot.com/23922004

git-svn-id: svn://svn.chromium.org/blink/trunk@157412 bbb929c8-8fbe-4397-9dbb-9b2b20218538