Revert "Use fMaxCharWidth as m_maxCharWidth on linux"

This reverts r160642 as it renders form controls a little too narrow
for certain fonts on linux.

BUG=322267
TBR=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/93433002

git-svn-id: svn://svn.chromium.org/blink/trunk@162798 bbb929c8-8fbe-4397-9dbb-9b2b20218538