DevTools: Add super.handleShortcut call to ElementsPanel.handleShortcut()

This fixes a search shortcut issue.

R=pfeldman, apavlov

Review URL: https://codereview.chromium.org/695323004

git-svn-id: svn://svn.chromium.org/blink/trunk@184907 bbb929c8-8fbe-4397-9dbb-9b2b20218538