Stop using Buffer::SetSize in some simple cases.

SetSize() can allow access to uninitialized memory, so should be
avoided. This CL addresses some simple cases, but does not yet
deprecate SetSize().

It also introduces Truncate() as a variant known not to expose
uninitialized memory.

Bug: webrtc:42223681
Change-Id: I136d2394139c7cb271686d634bf6a01edfd5691b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/449140
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46927}