Convert all webrtc code to not access EncodedImage::_size directly.

Read using capacity() method, write using set_buffer() method. This is
a preparation for making the member private, and renaming it to
capacity_.

Bug: webrtc:9378
Change-Id: I2f96679d052a83fe81be40301bd9863c87074640
Reviewed-on: https://webrtc-review.googlesource.com/c/113520
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@{#25934}
diff --git a/modules/video_coding/video_packet_buffer_unittest.cc b/modules/video_coding/video_packet_buffer_unittest.cc
index 8e0918c..ee890dd 100644
--- a/modules/video_coding/video_packet_buffer_unittest.cc
+++ b/modules/video_coding/video_packet_buffer_unittest.cc
@@ -647,7 +647,7 @@
   ASSERT_EQ(1UL, frames_from_callback_.size());
   EXPECT_EQ(frames_from_callback_[seq_num]->EncodedImage()._length,
             sizeof(data_data));
-  EXPECT_EQ(frames_from_callback_[seq_num]->EncodedImage()._size,
+  EXPECT_EQ(frames_from_callback_[seq_num]->EncodedImage().capacity(),
             sizeof(data_data) + EncodedImage::kBufferPaddingBytesH264);
   EXPECT_EQ(
       memcmp(frames_from_callback_[seq_num]->Buffer(), data, sizeof(data_data)),