Fix path tracer test.

Path tracer is not compatible with --enable-slow-assert.

R=mstarzinger@chromium.org
BUG=v8:3350
LOG=N

Review URL: https://codereview.chromium.org/297193005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21542 ce2b1a6d-e550-0410-aec6-3dcde31c8c00