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/encoded_frame.cc

@@ -39,7 +39,7 @@ void VCMEncodedFrame::Reset() {
   SetSpatialIndex(absl::nullopt);
   _renderTimeMs = -1;
   _payloadType = 0;
-  _frameType = kVideoFrameDelta;
+  _frameType = VideoFrameType::kVideoFrameDelta;
   _encodedWidth = 0;
   _encodedHeight = 0;
   _completeFrame = false;