Fix PathTracer.

When tracing, we abuse the map for marking, thereby mutating it.
FixedTypedArrayBase::size() uses the object's map, which causes crash.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/300753002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21523 ce2b1a6d-e550-0410-aec6-3dcde31c8c00