Fix build with GCC 4.7, which fails with "narrowing conversion of 'id' from 'int' to 'unsigned int' inside { } is ill-formed in C++11"

Contributed by burnus@net-b.de

Review URL: http://codereview.chromium.org/8724003
Patch from Tobias Burnus <burnus@net-b.de>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10101 ce2b1a6d-e550-0410-aec6-3dcde31c8c00