Use better heuristic for maxcharwidth.

Related skia change:
https://codereview.chromium.org/692083002/

BUG=420901

Review URL: https://codereview.chromium.org/718673004

git-svn-id: svn://svn.chromium.org/blink/trunk@185157 bbb929c8-8fbe-4397-9dbb-9b2b20218538