The code was using the "correct" cache key for lookups, but not for
creating new entries, leading to us never hitting the cache for
some Function-constructor cases.

Bug: v8:4958, chromium:801556, chromium:802400, chromium:807192
Change-Id: I4ac2234b97a9f5f71957ef936dc4b588d020916b
Reviewed-on: https://chromium-review.googlesource.com/898096Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51081}