Enable DirectWrite for LayoutTests

Enable the experimental DirectWrite font rendering backend for layout
tests on windows in preparation for enabling it by default later this
week.

This will require a very large number of rebaselines and I intent to
land it after hours and deal with the aftermath.

BUG=25541
R=cpu@chromium.org, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/300323003

git-svn-id: svn://svn.chromium.org/blink/trunk@175038 bbb929c8-8fbe-4397-9dbb-9b2b20218538