Change single-argument Buffer() to static create function.

This makes it more obvious when we're creating uninitialized data
that might be problematic later.

Bug: webrtc:42223681
Change-Id: Iabacade4170d8fb12d4fd3cf80dad301f7a5586e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/239842
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46566}