Rename IPC::Decoder::buffer() to span() for consistency with other types (such as IPC::Encoder).
Pull request: https://github.com/WebKit/WebKit/pull/28969
Committed 279208@main (a32a08ab27d7): <https://commits.webkit.org/279208@main> Reviewed commits have been landed. Closing PR #28969 and removing active labels.
<rdar://problem/128617593>