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/frame_buffer.cc b/modules/video_coding/frame_buffer.cc
index e1abb64..9be2ef0 100644
--- a/modules/video_coding/frame_buffer.cc
+++ b/modules/video_coding/frame_buffer.cc
@@ -105,12 +105,12 @@
       size() + packet.sizeBytes +
       (packet.insertStartCode ? kH264StartCodeLengthBytes : 0) +
       EncodedImage::GetBufferPaddingBytes(packet.codec);
-  if (requiredSizeBytes >= _size) {
+  if (requiredSizeBytes >= capacity()) {
     const uint8_t* prevBuffer = _buffer;
     const uint32_t increments =
         requiredSizeBytes / kBufferIncStepSizeBytes +
         (requiredSizeBytes % kBufferIncStepSizeBytes > 0);
-    const uint32_t newSize = _size + increments * kBufferIncStepSizeBytes;
+    const uint32_t newSize = capacity() + increments * kBufferIncStepSizeBytes;
     if (newSize > kMaxJBFrameSizeBytes) {
       RTC_LOG(LS_ERROR) << "Failed to insert packet due to frame being too "
                            "big.";