Remove unneccessary assert.

This assert fails when dumping native counters on x64.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/151033003

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19253 ce2b1a6d-e550-0410-aec6-3dcde31c8c00