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}
diff --git a/modules/video_coding/frame_buffer.cc b/modules/video_coding/frame_buffer.cc
index b623dd6..e1abb64 100644
--- a/modules/video_coding/frame_buffer.cc
+++ b/modules/video_coding/frame_buffer.cc
@@ -102,7 +102,7 @@
   }
 
   uint32_t requiredSizeBytes =
-      Length() + packet.sizeBytes +
+      size() + packet.sizeBytes +
       (packet.insertStartCode ? kH264StartCodeLengthBytes : 0) +
       EncodedImage::GetBufferPaddingBytes(packet.codec);
   if (requiredSizeBytes >= _size) {
@@ -139,7 +139,7 @@
     return kOutOfBoundsPacket;
   }
   // update length
-  _length = Length() + static_cast<uint32_t>(retVal);
+  _length = size() + static_cast<uint32_t>(retVal);
 
   _latestPacketTimeMs = timeInMs;