Revert of 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 dbb2743db1265890f4c7a3a854b6a415d059ee98..2d7e7f45653f98585026adc3044b1478d3ee1f87 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -30,6 +30,19 @@
 }
 
 }  // 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() {}