The checked node and used node were mismatched. It checked if
"var_unique" is a string, but it used "key" which may not be a string.

Bug: v8:4911, v8:7161, chromium:800077
Change-Id: Ia2aee8b77ac33500430365a4800bf9cca40a28fc
Reviewed-on: https://chromium-review.googlesource.com/855138Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50435}