Delete unused and almost unused frame-related methods.

webrtc::VideoFrame::set_video_frame_buffer
webrtc::VideoFrame::ConvertNativeToI420Frame

cricket::WebRtcVideoFrame::InitToBlack

VideoFrameBuffer::data
VideoFrameBuffer::stride
VideoFrameBuffer::MutableData

TBR=tkchin@webrtc.org # Refactoring affecting RTCVideoFrame
BUG=webrtc:5682

Review-Url: https://codereview.webrtc.org/2065733003
Cr-Commit-Position: refs/heads/master@{#13183}
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm b/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
index 872f6be..5b2d258 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
+++ b/webrtc/sdk/objc/Framework/Classes/RTCVideoFrame.mm
@@ -40,42 +40,42 @@
   if (!self.i420Buffer) {
     return nullptr;
   }
-  return self.i420Buffer->data(webrtc::kYPlane);
+  return self.i420Buffer->DataY();
 }
 
 - (const uint8_t *)uPlane {
   if (!self.i420Buffer) {
     return nullptr;
   }
-  return self.i420Buffer->data(webrtc::kUPlane);
+  return self.i420Buffer->DataU();
 }
 
 - (const uint8_t *)vPlane {
   if (!self.i420Buffer) {
     return nullptr;
   }
-  return self.i420Buffer->data(webrtc::kVPlane);
+  return self.i420Buffer->DataV();
 }
 
 - (int32_t)yPitch {
   if (!self.i420Buffer) {
     return 0;
   }
-  return self.i420Buffer->stride(webrtc::kYPlane);
+  return self.i420Buffer->StrideY();
 }
 
 - (int32_t)uPitch {
   if (!self.i420Buffer) {
     return 0;
   }
-  return self.i420Buffer->stride(webrtc::kUPlane);
+  return self.i420Buffer->StrideU();
 }
 
 - (int32_t)vPitch {
   if (!self.i420Buffer) {
     return 0;
   }
-  return self.i420Buffer->stride(webrtc::kVPlane);
+  return self.i420Buffer->StrideV();
 }
 
 - (int64_t)timeStamp {