Dropping the temporary StructPtr in the process.

Bug: v8:3770
Change-Id: I70784ede7b66b432d8438536ff0c70a51dfb7f83
Reviewed-on: https://chromium-review.googlesource.com/c/1377461
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58315}