Work around #4554 (Extension Manager font is hard to read on Windows), which
is a Chromium bug, by avoiding the lightest font-weight on Win. Lighten the
text slightly so it's still a little muted, like the design looks on Mac
and with older CEFs on Win.
Merge pull request #4566 from adobe/pflynn/windows-font-weight

Work around #4554 (Extension Manager font hard to read on Windows)