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/encoded_frame.h b/modules/video_coding/encoded_frame.h
index c7efd40..124ed44 100644
--- a/modules/video_coding/encoded_frame.h
+++ b/modules/video_coding/encoded_frame.h
@@ -65,21 +65,11 @@
    */
   uint8_t* MutableBuffer() { return _buffer; }
   /**
-   *   Get frame length
-   */
-  size_t Length() const { return _length; }
-  /**
-   *   Set frame length
-   */
-  void SetLength(size_t length) {
-    RTC_DCHECK(length <= _size);
-    _length = length;
-  }
-  /**
    *   Frame RTP timestamp (90kHz)
    */
   using EncodedImage::Timestamp;
   using EncodedImage::SetTimestamp;
+  using EncodedImage::size;
   /**
    *   Get render time in milliseconds
    */