tools: strip whitespace in gen-postmortem-metadata.py

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/221513002

Patch from Ben Noordhuis <ben@strongloop.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20436 ce2b1a6d-e550-0410-aec6-3dcde31c8c00