BuildNumberToString: Check for undefined keys in the cache

BUG=chromium:368114
LOG=y
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/302063004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21586 ce2b1a6d-e550-0410-aec6-3dcde31c8c00