Expose DirectWrite RuntimeEnabledFeature to WebRuntimeFeatures

Expose the DirectWrite RuntimeEnabledFeature to WebRuntimeFeatures to
allow it to be set from chrome. This in turn will allow us to add a
--direct-write command line flag for the experimental DirectWrite font
rendering support.

BUG=25541
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/71153006

git-svn-id: svn://svn.chromium.org/blink/trunk@161954 bbb929c8-8fbe-4397-9dbb-9b2b20218538