Add separate base classes for I420 and I444 buffers

webrtc/api/video/i420_buffer.h

Index: webrtc/api/video/i420_buffer.h
diff --git a/webrtc/api/video/i420_buffer.h b/webrtc/api/video/i420_buffer.h
index 20a84a1a53edae15ed9296717906abf6e8a3d197..c849ab22c053dbb63dfa76bb2f3fac1785dba7f7 100644
--- a/webrtc/api/video/i420_buffer.h
+++ b/webrtc/api/video/i420_buffer.h
@@ -20,7 +20,7 @@
 namespace webrtc {
 
 // Plain I420 buffer in standard memory.
-class I420Buffer : public PlanarYuvBuffer {
+class I420Buffer : public I420BufferInterface {
  public:
   static rtc::scoped_refptr<I420Buffer> Create(int width, int height);
   static rtc::scoped_refptr<I420Buffer> Create(int width,
@@ -53,7 +53,6 @@ class I420Buffer : public PlanarYuvBuffer {
   // are resolved in a better way. Or in the mean time, use SetBlack.
   void InitializeData();
 
-  Type type() const override;
   int width() const override;
   int height() const override;
   const uint8_t* DataY() const override;