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}
