Fix Mac avgCharWidth and maxCharWidth calculation

The current avgCharWidth and maxCharWidth calculation using the values provided by FontMetrics doesn't work yet on Mac. Revert back to old Mac-specific behavior.

BUG=420049
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/594983003

git-svn-id: svn://svn.chromium.org/blink/trunk@183309 bbb929c8-8fbe-4397-9dbb-9b2b20218538