DevTools: [CodeMirror] use Tab to accept autocompletion

This patch adds a functionality to commit autocomplete suggestion via
Tab key.

BUG=261560

Review URL: https://chromiumcodereview.appspot.com/19558004

git-svn-id: svn://svn.chromium.org/blink/trunk@154573 bbb929c8-8fbe-4397-9dbb-9b2b20218538