DevTools: [CodeMirror] add overlay.js to devtools.gypi
DevTools: [CodeMirror] add overlay.js to devtools.gypi

Codemirror addon overlay.js was introduced in r151821 and by mistake
was not added to the gypi file.

BUG=
R=apavlov@chromium.org, vsevik@chromium.org

Review URL: https://codereview.chromium.org/16416003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@151824 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs