fix(test): Handle worktrees in repo root detection scripts

The scripts assumed the repo root was named 'src' or 'chromium'. This failed when running in a worktree with a different name. Changed to look for the 'DEPS' file instead, which is more robust.

Affected files:

- tools_webrtc/validate_test_registration.py

- tools_webrtc/presubmit_checks_lib/build_helpers.py

- tools_webrtc/autoroller/roll_deps.py

Bug: None
Change-Id: Ide6a800456ef4e279d0a292d8391f38337cc838c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/477460
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47903}