DevTools: fix execution line update on pretty-print

This patch resets old execution line location on pretty-printing as it
will be set later as soon as it's live location will be updated.

BUG=334188

Review URL: https://codereview.chromium.org/131743005

git-svn-id: svn://svn.chromium.org/blink/trunk@165089 bbb929c8-8fbe-4397-9dbb-9b2b20218538