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}
diff --git a/modules/video_coding/frame_buffer.cc b/modules/video_coding/frame_buffer.cc
index 9be2ef0..908a2a2 100644
--- a/modules/video_coding/frame_buffer.cc
+++ b/modules/video_coding/frame_buffer.cc
@@ -138,8 +138,8 @@
   } else if (retVal == -3) {
     return kOutOfBoundsPacket;
   }
-  // update length
-  _length = size() + static_cast<uint32_t>(retVal);
+  // update size
+  set_size(size() + static_cast<uint32_t>(retVal));
 
   _latestPacketTimeMs = timeInMs;
 
@@ -216,7 +216,7 @@
 
 void VCMFrameBuffer::Reset() {
   TRACE_EVENT0("webrtc", "VCMFrameBuffer::Reset");
-  _length = 0;
+  set_size(0);
   _sessionInfo.Reset();
   _payloadType = 0;
   _nackCount = 0;
@@ -265,7 +265,7 @@
 void VCMFrameBuffer::PrepareForDecode(bool continuous) {
   TRACE_EVENT0("webrtc", "VCMFrameBuffer::PrepareForDecode");
   size_t bytes_removed = _sessionInfo.MakeDecodable();
-  _length -= bytes_removed;
+  set_size(size() - bytes_removed);
   // Transfer frame information to EncodedFrame and create any codec
   // specific information.
   _frameType = _sessionInfo.FrameType();