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_buffer2_unittest.cc b/modules/video_coding/frame_buffer2_unittest.cc
index 321281f..94a5551 100644
--- a/modules/video_coding/frame_buffer2_unittest.cc
+++ b/modules/video_coding/frame_buffer2_unittest.cc
@@ -91,10 +91,6 @@
   int64_t ReceivedTime() const override { return 0; }
 
   int64_t RenderTime() const override { return _renderTimeMs; }
-
-  // In EncodedImage |_length| is used to descibe its size and |_size| to
-  // describe its capacity.
-  void SetSize(int size) { _length = size; }
 };
 
 class VCMReceiveStatisticsCallbackMock : public VCMReceiveStatisticsCallback {
@@ -168,7 +164,7 @@
     frame->is_last_spatial_layer = last_spatial_layer;
     // Add some data to buffer.
     frame->VerifyAndAllocate(kFrameSize);
-    frame->SetSize(kFrameSize);
+    frame->set_size(kFrameSize);
     for (size_t r = 0; r < references.size(); ++r)
       frame->references[r] = references[r];
 
@@ -491,7 +487,7 @@
   {
     std::unique_ptr<FrameObjectFake> frame(new FrameObjectFake());
     frame->VerifyAndAllocate(kFrameSize);
-    frame->SetSize(kFrameSize);
+    frame->set_size(kFrameSize);
     frame->id.picture_id = pid;
     frame->id.spatial_layer = 0;
     frame->SetTimestamp(ts);