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/sdk/objc/api/peerconnection/RTCEncodedImage+Private.mm b/sdk/objc/api/peerconnection/RTCEncodedImage+Private.mm
index 6f2d1f4..59962c8 100644
--- a/sdk/objc/api/peerconnection/RTCEncodedImage+Private.mm
+++ b/sdk/objc/api/peerconnection/RTCEncodedImage+Private.mm
@@ -17,9 +17,9 @@
 - (instancetype)initWithNativeEncodedImage:(webrtc::EncodedImage)encodedImage {
   if (self = [super init]) {
     // Wrap the buffer in NSData without copying, do not take ownership.
-    self.buffer = [NSData dataWithBytesNoCopy:encodedImage._buffer
-                                   length:encodedImage._length
-                             freeWhenDone:NO];
+    self.buffer = [NSData dataWithBytesNoCopy:encodedImage.data()
+                                       length:encodedImage.size()
+                                 freeWhenDone:NO];
     self.encodedWidth = rtc::dchecked_cast<int32_t>(encodedImage._encodedWidth);
     self.encodedHeight = rtc::dchecked_cast<int32_t>(encodedImage._encodedHeight);
     self.timeStamp = encodedImage.Timestamp();