DevTools: Fix text going out when adding a new Watch Expression.

BUG=338654
R=apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/178543014

git-svn-id: svn://svn.chromium.org/blink/trunk@168381 bbb929c8-8fbe-4397-9dbb-9b2b20218538