DevTools: [Sources] fix conditional breakpoint text label

There was an off-by-one error which this patch fixes.

BUG=370928

Review URL: https://codereview.chromium.org/280433003

git-svn-id: svn://svn.chromium.org/blink/trunk@173852 bbb929c8-8fbe-4397-9dbb-9b2b20218538