DevTools: [Styles] Fix warning icon showing a part of a sprite to its right

R=pfeldman, vsevik
BUG=319230

Review URL: https://codereview.chromium.org/62953021

git-svn-id: svn://svn.chromium.org/blink/trunk@162012 bbb929c8-8fbe-4397-9dbb-9b2b20218538