Revert "Update webrtc/sdk/objc to new VideoFrameBuffer interface"

This reverts commit b008b45f1e609556a04c1aabb4e8ed6a894265af.

Reason for revert: Breaks external clients.

Original change's description:
> Update webrtc/sdk/objc to new VideoFrameBuffer interface
> 
> More thorough refactoring work is planned for RTCVideoFrame (see webrtc:7785), and this CL just unblocks removing the old interface from webrtc::VideoFrameBuffer.
> 
> Bug: webrtc:7632,webrtc:7785
> Change-Id: I351536c5ca454c2acd8944bbc2ebb1d1439dc50c
> Reviewed-on: https://chromium-review.googlesource.com/530231
> Reviewed-by: Anders Carlsson <andersc@webrtc.org>
> Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#18553}

TBR=magjed@webrtc.org,andersc@webrtc.org
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:7632,webrtc:7785

Change-Id: Ib5c6fcb939175c67c3ac7b3df7cea0f7c2bb0af0
Reviewed-on: https://chromium-review.googlesource.com/533013
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18557}

webrtc/sdk/objc/Framework/Classes/PeerConnection/RTCVideoFrame.mm

@@ -31,27 +31,27 @@
 }
 
 - (const uint8_t *)dataY {
-  return _videoBuffer->GetI420()->DataY();
+  return _videoBuffer->DataY();
 }
 
 - (const uint8_t *)dataU {
-  return _videoBuffer->GetI420()->DataU();
+  return _videoBuffer->DataU();
 }
 
 - (const uint8_t *)dataV {
-  return _videoBuffer->GetI420()->DataV();
+  return _videoBuffer->DataV();
 }
 
 - (int)strideY {
-  return _videoBuffer->GetI420()->StrideY();
+  return _videoBuffer->StrideY();
 }
 
 - (int)strideU {
-  return _videoBuffer->GetI420()->StrideU();
+  return _videoBuffer->StrideU();
 }
 
 - (int)strideV {
-  return _videoBuffer->GetI420()->StrideV();
+  return _videoBuffer->StrideV();
 }
 
 - (int64_t)timeStampNs {
@@ -59,14 +59,12 @@
 }
 
 - (CVPixelBufferRef)nativeHandle {
-  return (_videoBuffer->type() == webrtc::VideoFrameBuffer::Type::kNative) ?
-      static_cast<webrtc::CoreVideoFrameBuffer *>(_videoBuffer.get())->pixel_buffer() :
-      nil;
+  return static_cast<CVPixelBufferRef>(_videoBuffer->native_handle());
 }
 
 - (RTCVideoFrame *)newI420VideoFrame {
   return [[RTCVideoFrame alloc]
-      initWithVideoBuffer:_videoBuffer->ToI420()
+      initWithVideoBuffer:_videoBuffer->NativeToI420Buffer()
                  rotation:_rotation
               timeStampNs:_timeStampNs];
 }