Do not use out of range visitor ids.

BUG=

Review URL: https://codereview.chromium.org/822693002

Cr-Commit-Position: refs/heads/master@{#25920}
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}