HeapProfiler: compile fix for win x64.

BUG=none
TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/24178003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16738 ce2b1a6d-e550-0410-aec6-3dcde31c8c00