Fix unsigned comparison in previousIfPositionIsAfterLineBreak

Fix unsigned comparison in previousIfPositionIsAfterLineBreak, the unsigned can't be negative so don't test for it.

Review URL: https://codereview.chromium.org/395253002

git-svn-id: svn://svn.chromium.org/blink/trunk@178312 bbb929c8-8fbe-4397-9dbb-9b2b20218538