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

webrtc/common_video/include/video_frame_buffer.h

Index: webrtc/common_video/include/video_frame_buffer.h
diff --git a/webrtc/common_video/include/video_frame_buffer.h b/webrtc/common_video/include/video_frame_buffer.h
index ae7855fe86d5aa3dfcc952617eed0b89f802c376..674397020369a92b55bac8668c04a645d1c2e68c 100644
--- a/webrtc/common_video/include/video_frame_buffer.h
+++ b/webrtc/common_video/include/video_frame_buffer.h
@@ -45,6 +45,12 @@
   virtual const uint8_t* DataU() const = 0;
   virtual const uint8_t* DataV() const = 0;
 
+  // TODO(nisse): Move MutableData methods to the I420Buffer subclass.
+  // Non-const data access.
+  virtual uint8_t* MutableDataY();
+  virtual uint8_t* MutableDataU();
+  virtual uint8_t* MutableDataV();
+
   // Returns the number of bytes between successive rows for a given plane.
   virtual int StrideY() const = 0;
   virtual int StrideU() const = 0;
@@ -92,9 +98,9 @@
   const uint8_t* DataU() const override;
   const uint8_t* DataV() const override;
 
-  uint8_t* MutableDataY();
-  uint8_t* MutableDataU();
-  uint8_t* MutableDataV();
+  uint8_t* MutableDataY() override;
+  uint8_t* MutableDataU() override;
+  uint8_t* MutableDataV() override;
   int StrideY() const override;
   int StrideU() const override;
   int StrideV() const override;