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

webrtc/common_video/i420_buffer_pool_unittest.cc

Index: webrtc/common_video/i420_buffer_pool_unittest.cc
diff --git a/webrtc/common_video/i420_buffer_pool_unittest.cc b/webrtc/common_video/i420_buffer_pool_unittest.cc
index 3e795dbdc4b3f021ca77ef3122b570bb843a5172..3307539c4e7744762d2c5357de405be7b9744ec1 100644
--- a/webrtc/common_video/i420_buffer_pool_unittest.cc
+++ b/webrtc/common_video/i420_buffer_pool_unittest.cc
@@ -52,7 +52,7 @@ TEST(TestI420BufferPool, FailToReuse) {
 }
 
 TEST(TestI420BufferPool, FrameValidAfterPoolDestruction) {
-  rtc::scoped_refptr<VideoFrameBuffer> buffer;
+  rtc::scoped_refptr<I420Buffer> buffer;
   {
     I420BufferPool pool;
     buffer = pool.CreateBuffer(16, 16);