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}

sdk/objc/Framework/UnitTests/frame_buffer_helpers.h

@@ -0,0 +1,22 @@
+/*
+ *  Copyright 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import <AVFoundation/AVFoundation.h>
+
+#include "api/video/i420_buffer.h"
+
+void DrawGradientInRGBPixelBuffer(CVPixelBufferRef pixelBuffer);
+
+rtc::scoped_refptr<webrtc::I420Buffer> CreateI420Gradient(int width,
+                                                          int height);
+
+void CopyI420BufferToCVPixelBuffer(
+    rtc::scoped_refptr<webrtc::I420Buffer> i420Buffer,
+    CVPixelBufferRef pixelBuffer);