Spanify rtc_base/buffer

This makes it possible to compile rtc_base/buffer with the checks
for unsafe buffers.

Bug: webrtc:478086887
Change-Id: I53f1161164e0408f985bd8395a0fd516e47bb0cd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/444780
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46858}