No functional change. This makes the code more generic and this reduces the size of screen_capturer_mac.mm Bug: webrtc:8652 Change-Id: I37743ba385fea5e1b40df3b094bfc321b8d796ae Reviewed-on: https://webrtc-review.googlesource.com/65082 Commit-Queue: Julien Isorce <julien.isorce@chromium.org> Commit-Queue: Zijie He <zijiehe@chromium.org> Reviewed-by: Zijie He <zijiehe@chromium.org> Cr-Commit-Position: refs/heads/master@{#22684}