Reland "Add unit tests for RTCCVPixelBuffer and ObjCVideoTrackSource."

This is a reland of 4ea50c2b421ae3e40d1d02b8eb8c5802288b181e

Original change's description:
> Add unit tests for RTCCVPixelBuffer and ObjCVideoTrackSource.
> 
> This CL also fixes a couple of bugs found in the toI420 method for
> RTCCVPixelBuffers backed by RGB CVPixelBuffers.
> 
> Bug: webrtc:9007
> Change-Id: I19ab8177f4b124a503cfda9f0166bd960f668982
> Reviewed-on: https://webrtc-review.googlesource.com/64940
> Commit-Queue: Anders Carlsson <andersc@webrtc.org>
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#22656}

Bug: webrtc:9007
Change-Id: I2a787c64f8d23ffc4ef2419fc258d965f8a9480b
Reviewed-on: https://webrtc-review.googlesource.com/66341
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22706}
diff --git a/sdk/objc/Framework/Classes/Video/RTCI420Buffer.mm b/sdk/objc/Framework/Classes/Video/RTCI420Buffer.mm
index a428a8d..2848f2b 100644
--- a/sdk/objc/Framework/Classes/Video/RTCI420Buffer.mm
+++ b/sdk/objc/Framework/Classes/Video/RTCI420Buffer.mm
@@ -89,6 +89,10 @@
   return self;
 }
 
+- (rtc::scoped_refptr<webrtc::I420BufferInterface>)nativeI420Buffer {
+  return _i420Buffer;
+}
+
 @end
 
 @implementation RTCMutableI420Buffer