Do not propagate generic descriptor on receiving frame

It was used only for the frame decryptor.
Decryptor needs only raw representation that it can recreate
in a way compatible with the new version of the descriptor.

This relands commit abf73de8eae90e9ac7e88ce1d52728e8102e824f.
with adjustments.

Change-Id: I935977179bef31d8e1023964b967658e9a7db92d
Bug: webrtc:10342
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168489
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30532}

modules/video_coding/rtp_frame_reference_finder_unittest.cc

@@ -57,7 +57,6 @@ std::unique_ptr<RtpFrameObject> CreateFrame(
       VideoContentType::UNSPECIFIED,
       video_header,
       /*color_space=*/absl::nullopt,
-      /*generic_descriptor=*/absl::nullopt,
       RtpPacketInfos(),
       EncodedImageBuffer::Create(/*size=*/0));
   // clang-format on