Fix windows build (size_t vs. int).

Sadly, Vector ctor takes an argument "int length".

TBR=dcarney@chromium.org, mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/334903002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21844 ce2b1a6d-e550-0410-aec6-3dcde31c8c00