DevTools: Introduce UIStrings for normally user-visible UI strings

The patch does not cover exception or console.assert messages.

R=aandrey, eustas, pfeldman, vsevik

Review URL: https://codereview.chromium.org/131783006

git-svn-id: svn://svn.chromium.org/blink/trunk@165319 bbb929c8-8fbe-4397-9dbb-9b2b20218538