Work-around to make webkit test suite work with optdebug==2.

The test driver doesn't know at the moment if optdebug 1 or 2 has been used (the first is used on the bots, the second
locally in quickcheck). It can't know therefore if slow
asserts can be enabled or not. Trying to enable them leads
to a warning if they weren't compiled, which causes webkit
1:1 text comparison to fail.

Review URL: https://codereview.chromium.org/819033002

Cr-Commit-Position: refs/heads/master@{#25921}

test/webkit/testcfg.py

@@ -109,7 +109,11 @@ class WebkitTestSuite(testsuite.TestSuite):
             string.startswith("tools/nacl-run.py") or
             string.find("BYPASSING ALL ACL CHECKS") > 0 or
             string.find("Native Client module will be loaded") > 0 or
-            string.find("NaClHostDescOpen:") > 0)
+            string.find("NaClHostDescOpen:") > 0 or
+            # FIXME(machenbach): The test driver shouldn't try to use slow
+            # asserts if they weren't compiled. This fails in optdebug=2.
+            string == "Warning: unknown flag --enable-slow-asserts." or
+            string == "Try --help for options")
 
   def IsFailureOutput(self, output, testpath):
     if super(WebkitTestSuite, self).IsFailureOutput(output, testpath):