Allow default font size changing on Linux.


See https://codereview.chromium.org/549303004/ for Chromium part.


BUG=

Review URL: https://codereview.chromium.org/570243002

git-svn-id: svn://svn.chromium.org/blink/trunk@183350 bbb929c8-8fbe-4397-9dbb-9b2b20218538