Move RtpFrameObject and EncodedFrame out of video_coding namespace.

Bug: webrtc:12579
Change-Id: Ib7ecd624eb5c54abb77fe08440a014aa1e963865
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212860
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33542}

video/video_stream_decoder_impl_unittest.cc

@@ -127,7 +127,7 @@ class FakeVideoDecoderFactory : public VideoDecoderFactory {
   NiceMock<StubVideoDecoder> av1_decoder_;
 };
 
-class FakeEncodedFrame : public video_coding::EncodedFrame {
+class FakeEncodedFrame : public EncodedFrame {
  public:
   int64_t ReceivedTime() const override { return 0; }
   int64_t RenderTime() const override { return 0; }