Rename EncodedImage::_length --> size_, and make private.

Use size() accessor function. Also replace most nearby uses of _buffer
with data().

Bug: webrtc:9378
Change-Id: I1ac3459612f7c6151bd057d05448da1c4e1c6e3d
Reviewed-on: https://webrtc-review.googlesource.com/c/116783
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26273}

modules/video_coding/encoded_frame.cc

@@ -52,7 +52,7 @@ void VCMEncodedFrame::Reset() {
   _encodedHeight = 0;
   _completeFrame = false;
   _missingFrame = false;
-  _length = 0;
+  set_size(0);
   _codecSpecificInfo.codecType = kVideoCodecGeneric;
   _codec = kVideoCodecGeneric;
   rotation_ = kVideoRotation_0;