Fix cast_unittest warnings on win_x64_rel

Fix warnings with size_t casted to int.

TBR=justinlin@chromium.org

Review URL: https://codereview.chromium.org/26034003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227051 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227051