DevTools: fix pretty-print functionality for VM scripts

This patch fixes pretty-print functionality for VM scripts.

BUG=356269

Review URL: https://codereview.chromium.org/211243006

git-svn-id: svn://svn.chromium.org/blink/trunk@170056 bbb929c8-8fbe-4397-9dbb-9b2b20218538