Make VideoFrameType an enum class, and move to separate file and target

Bug: webrtc:5876, webrtc:6883
Change-Id: I1435cfa9e8e54c4ba2978261048ff3fbb993ce0e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/126225
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27239}

modules/video_coding/test/stream_generator.cc

@@ -44,7 +44,7 @@ void StreamGenerator::GenerateFrame(VideoFrameType type,
   }
   for (int i = 0; i < num_empty_packets; ++i) {
     packets_.push_back(GeneratePacket(sequence_number_, timestamp, 0, false,
-                                      false, kEmptyFrame));
+                                      false, VideoFrameType::kEmptyFrame));
     ++sequence_number_;
   }
 }