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/video_packet_buffer_unittest.cc b/modules/video_coding/video_packet_buffer_unittest.cc
index 49b1c26..190e0e7 100644
--- a/modules/video_coding/video_packet_buffer_unittest.cc
+++ b/modules/video_coding/video_packet_buffer_unittest.cc
@@ -643,7 +643,7 @@
   packet_buffer_->InsertPacket(&packet);
 
   ASSERT_EQ(1UL, frames_from_callback_.size());
-  EXPECT_EQ(frames_from_callback_[seq_num]->EncodedImage()._length,
+  EXPECT_EQ(frames_from_callback_[seq_num]->EncodedImage().size(),
             sizeof(data_data));
   EXPECT_EQ(frames_from_callback_[seq_num]->EncodedImage().capacity(),
             sizeof(data_data) + EncodedImage::kBufferPaddingBytesH264);