This CL adds an EncodedFrameChecker interface which can be used by users of the VideoProcessor to inject customized per-frame checks to the encoding/decoding pipeline. This currently has two uses: - Verifying that the QP parser works correctly for VP8 and VP9, by comparing the parsed QP to that produced by libvpx. - Verifying that our H.264 encoders always produce SPS/PPS/IDR in tandem. TESTED=Galaxy S8, Pixel 2 XL, iPhone 7. BUG=webrtc:8423 Change-Id: Ic3e401546e239a9ffaf2ed2907689cebb1127805 Reviewed-on: https://webrtc-review.googlesource.com/14559 Reviewed-by: Åsa Persson <asapersson@webrtc.org> Reviewed-by: Sergey Silkin <ssilkin@webrtc.org> Commit-Queue: Rasmus Brandt <brandtr@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20409}