Limit synthetic bold/italic webfonts workaround to GDI skia backend

Limit the workaround for synthetic bold and/or italic webfonts to the
Skia GDI backend as it does not apply to the DirectWrite backend.

This ensures that synthetic bold and/or italic webfonts render correctly
regardless of skia font backend on Windows.

BUG=333430
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/266213003

git-svn-id: svn://svn.chromium.org/blink/trunk@173344 bbb929c8-8fbe-4397-9dbb-9b2b20218538