Fix spurious debug-script assertion after enabling Symbols

This approach, of allowing either N or N+1 in this assert,
is the same approach used before my Symbols patch that naively
changed to use assertEquals.

TBR=dslomov@chromium.org

Review URL: https://codereview.chromium.org/439833002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22834 ce2b1a6d-e550-0410-aec6-3dcde31c8c00