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/jitter_buffer_unittest.cc b/modules/video_coding/jitter_buffer_unittest.cc
index 3ed18a9..e95a312 100644
--- a/modules/video_coding/jitter_buffer_unittest.cc
+++ b/modules/video_coding/jitter_buffer_unittest.cc
@@ -283,7 +283,7 @@
       i += 4;
     }
 
-    EXPECT_EQ(size, frame_out->Length());
+    EXPECT_EQ(size, frame_out->size());
     int count = 3;
     for (; i < size; i++) {
       if (outData[i] == 0 && outData[i + 1] == 0 && outData[i + 2] == 0x80) {