[Windows] Fall back on 2 x ascent if no max width is present 

If the max character width cannot be determined for a font fall back on
2 x the font ascent.

R=bungeman@chromium.org
BUG=395043

Review URL: https://codereview.chromium.org/413033002

git-svn-id: svn://svn.chromium.org/blink/trunk@178812 bbb929c8-8fbe-4397-9dbb-9b2b20218538