fix for NPVariant numbers. (new webkit v8 bindings give us doubles for ints)

Issue: http://code.google.com/p/google-web-toolkit/issues/detail?id=5778
Breaking WebKit change: http://trac.webkit.org/changeset/72974

Review at http://gwt-code-reviews.appspot.com/1277801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9525 8db76d5a-ed1c-0410-87a9-c151d255dfc7