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/frame_buffer.cc

@@ -94,7 +94,7 @@ VCMFrameBufferEnum VCMFrameBuffer::InsertPacket(
     // We only take the ntp timestamp of the first packet of a frame.
     ntp_time_ms_ = packet.ntp_time_ms_;
     _codec = packet.codec();
-    if (packet.frameType != kEmptyFrame) {
+    if (packet.frameType != VideoFrameType::kEmptyFrame) {
       // first media packet
       SetState(kStateIncomplete);
     }