fuzz natives times out on some machines due to DebugReferencedBy

Review URL: http://codereview.chromium.org/9039

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@677 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

test/mjsunit/fuzz-natives.js

@@ -107,6 +107,9 @@ var knownProblems = {
   "ChangeBreakOnException": true,
   "PrepareStep": true,
 
+  // Too slow.
+  "DebugReferencedBy": true,
+
   // Calling disable/enable access checks may interfere with the
   // the rest of the tests.
   "DisableAccessChecks": true,