DevTools: Extract a common string label to UIUtils.

R=vsevik@chromium.org, apavlov, vsevik

Review URL: https://codereview.chromium.org/342133003

git-svn-id: svn://svn.chromium.org/blink/trunk@176600 bbb929c8-8fbe-4397-9dbb-9b2b20218538