Ignore numbers as values of --expose-natives-as flag.

R=yangguo@chromium.org
BUG=408036
LOG=N

Review URL: https://codereview.chromium.org/534943004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23700 ce2b1a6d-e550-0410-aec6-3dcde31c8c00