Fix type in idle time undershot histogram name

TBR=hpayer@chromium.org
LOG=n
BUG=none

Review URL: https://codereview.chromium.org/587563003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24071 ce2b1a6d-e550-0410-aec6-3dcde31c8c00