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.cc

@@ -135,6 +135,11 @@ int64_t RtpFrameObject::RenderTime() const {
   return _renderTimeMs;
 }
 
+void RtpFrameObject::SetSize(size_t size) {
+  RTC_DCHECK_LE(size, _size);
+  _length = size;
+}
+
 bool RtpFrameObject::delayed_by_retransmission() const {
   return times_nacked() > 0;
 }