Don't clear EnumLength but rather copy the enum cache. Added regression test for crashes from chromecrash.

Review URL: https://chromiumcodereview.appspot.com/11103036

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12704 ce2b1a6d-e550-0410-aec6-3dcde31c8c00