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

webrtc/common_video/i420_buffer_pool_unittest.cc

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 34 matching lines @@
   buffer = nullptr;
   // Check that the pool doesn't try to reuse buffers of incorrect size.
   buffer = pool.CreateBuffer(32, 16);
   EXPECT_EQ(32, buffer->width());
   EXPECT_EQ(16, buffer->height());
   EXPECT_NE(u_ptr, buffer->DataU());
   EXPECT_NE(v_ptr, buffer->DataV());
 }
 
 TEST(TestI420BufferPool, FrameValidAfterPoolDestruction) {
-  rtc::scoped_refptr<VideoFrameBuffer> buffer;
+  rtc::scoped_refptr<I420Buffer> buffer;
   {
     I420BufferPool pool;
     buffer = pool.CreateBuffer(16, 16);
   }
   EXPECT_EQ(16, buffer->width());
   EXPECT_EQ(16, buffer->height());
   // Try to trigger use-after-free errors by writing to y-plane.
   memset(buffer->MutableDataY(), 0xA5, 16 * buffer->StrideY());
 }
 
 }  // namespace webrtc