Fix -Wconversion warnings in external V8 headers.

BUG=v8:2830
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/22549003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16127 ce2b1a6d-e550-0410-aec6-3dcde31c8c00