Allow absl::Nonnull and absl::Nullable.

This CL includes follow-up changes from
https://webrtc-review.googlesource.com/c/src/+/324280

Bug: none
Change-Id: I6abad16e05cac7197c51ffa7b1d3fb991843df6e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/325243
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41030}
diff --git a/DEPS b/DEPS
index a98e19d..4575918 100644
--- a/DEPS
+++ b/DEPS
@@ -2602,6 +2602,7 @@
   "+absl/base/attributes.h",
   "+absl/base/config.h",
   "+absl/base/const_init.h",
+  "+absl/base/nullability.h",
   "+absl/base/macros.h",
   "+absl/cleanup/cleanup.h",
   "+absl/container/inlined_vector.h",