It seems that I forgot to remove the DCHECK when refactoring this
function, even though the comment had it right.  It also seems that
this is hard to trigger.  The minimal example I found, after fuzzer's
bug, was:

   eval, x[eval]

R=adamk@chromium.org
BUG=chromium:619476
LOG=N

Review-Url: https://codereview.chromium.org/2058413002
Cr-Commit-Position: refs/heads/master@{#36929}