Fix synthetic bold/italic for webfonts on windows

Add workaround for rendering synthetic bold/italic for webfonts on
windows. The current skia implementation for synthetic bold/italic
on windows results in pretty bad rendering, especially for
synthetic bold. With this workaround in place the rendering matches
the rendering in previous versions of Chrome and other browsers.

TEST=fast/text/webfont-synthetic-bold.html
BUG=332958
R=bungeman@chromium.org,dglazkov@chromium.org

Review URL: https://codereview.chromium.org/135903012

git-svn-id: svn://svn.chromium.org/blink/trunk@165359 bbb929c8-8fbe-4397-9dbb-9b2b20218538