[Extensions] Upgrade render extension registry insert to CHECK().

I couldn't find any crashes with this signature, so upgrading to CHECK()
from the TODO comment.

Bug: 40434302
Change-Id: I02cd5d33eb7f61bdca280345cb622ef1a385cdba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5787473
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1341931}