DevTools: [CodeMirror] center breakpoints in execution line

This patch centers breakpoints in execution line.

BUG=
TBR=pfeldman

Review URL: https://codereview.chromium.org/21094002

git-svn-id: svn://svn.chromium.org/blink/trunk@155087 bbb929c8-8fbe-4397-9dbb-9b2b20218538