kIvfHeaderSize is defined both inside of ivf_file_writer.cc and ivf_file_reader.cc. This patch moves its definition into a header. Bug: webrtc:13463 Change-Id: Ia6b2fcc3434f69a1e30a7dae7bf0c90547f11d98 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/239722 Reviewed-by: Harald Alvestrand <hta@webrtc.org> Commit-Queue: Harald Alvestrand <hta@webrtc.org> Cr-Commit-Position: refs/heads/main@{#35540}