For font weights >= 600, create bold system fonts.

Adjust the weight threshold used for creating bold system fonts down
to 600, so as align with the weight matching specified by

  http://www.w3.org/TR/CSS21/fonts.html#propdef-font-weight

It also aligns better with others.

R=eae@chromium.org
BUG=335050

Review URL: https://codereview.chromium.org/387873002

git-svn-id: svn://svn.chromium.org/blink/trunk@177999 bbb929c8-8fbe-4397-9dbb-9b2b20218538