...mainly by giving a more precise type to global_proxy getters.

Change-Id: If4aef6b25baa2c641a45b177c59690e3ebfc3985
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505578
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60072}