Add separate base classes for I420 and I444 buffers

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 0322dd1fd506027d4529464871ffbc0dca3393d1..5f339a5f35e70559dd80a16c610e7df91601450c 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -96,10 +96,6 @@ WrappedI420Buffer::~WrappedI420Buffer() {
   no_longer_used_cb_();
 }
 
-VideoFrameBuffer::Type WrappedI420Buffer::type() const {
-  return Type::kI420;
-}
-
 int WrappedI420Buffer::width() const {
   return width_;
 }