Drive-by: add const modifier to some StructType fields

Bug: v8:7748
Change-Id: Ic0b4bb51ed01502f19d082c669683f69b85e76e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2419015
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70029}