Fix --expose-debug-as with number as argument.

R=jkummerow@chromium.org
BUG=405491
LOG=N

Review URL: https://codereview.chromium.org/468803004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23228 ce2b1a6d-e550-0410-aec6-3dcde31c8c00