Turn iterator .end() DCHECK() into CHECK() for headless

Accessing an invalid iterator can sometimes be a security
issue and these checks are cheap, so upgrade to CHECKs.

For this CL instances were located with a weggli query
of `DCHECK(_ != _.end());` and some filtering of paths.

This CL was uploaded by git cl split.

R=eseckler@chromium.org

Bug: 351745839
Change-Id: I24f4984715c1af7980732013bd18d8819cf0fb10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5685161
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Auto-Submit: Alex Gough <ajgo@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1324749}