Move size() method to EncodedImage base class

Deleted from subclass video_coding::EncodedFrame. Also delete Length
and SetLength methods on the intermediate class
video_coding::VCMEncodedFrame.

Bug: webrtc:9378
Change-Id: I3c90b14735f622f50b2f403f79072e22fc025d11
Reviewed-on: https://webrtc-review.googlesource.com/c/112131
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25838}

modules/video_coding/frame_object.h

@@ -39,6 +39,7 @@ class RtpFrameObject : public EncodedFrame {
   VideoCodecType codec_type() const;
   int64_t ReceivedTime() const override;
   int64_t RenderTime() const override;
+  void SetSize(size_t size);
   bool delayed_by_retransmission() const override;
   absl::optional<RTPVideoHeader> GetRtpVideoHeader() const;
   absl::optional<RtpGenericFrameDescriptor> GetGenericFrameDescriptor() const;