Move MutableDataY{,U,V} methods to I420Buffer only.

webrtc/common_video/video_frame_buffer.cc

Index: webrtc/common_video/video_frame_buffer.cc
diff --git a/webrtc/common_video/video_frame_buffer.cc b/webrtc/common_video/video_frame_buffer.cc
index 2d7e7f45653f98585026adc3044b1478d3ee1f87..dbb2743db1265890f4c7a3a854b6a415d059ee98 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -31,19 +31,6 @@ int I420DataSize(int height, int stride_y, int stride_u, int stride_v) {
 
 }  // namespace
 
-uint8_t* VideoFrameBuffer::MutableDataY() {
-  RTC_NOTREACHED();
-  return nullptr;
-}
-uint8_t* VideoFrameBuffer::MutableDataU() {
-  RTC_NOTREACHED();
-  return nullptr;
-}
-uint8_t* VideoFrameBuffer::MutableDataV() {
-  RTC_NOTREACHED();
-  return nullptr;
-}
-
 VideoFrameBuffer::~VideoFrameBuffer() {}
 
 I420Buffer::I420Buffer(int width, int height)