VideoFrameBuffer: Make non-const data access explicit

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 50 matching lines @@
 TEST(TestI420BufferPool, FrameValidAfterPoolDestruction) {
   rtc::scoped_refptr<VideoFrameBuffer> buffer;
   {
     I420BufferPool pool;
     buffer = pool.CreateBuffer(16, 16);
   }
   EXPECT_TRUE(buffer->HasOneRef());
   EXPECT_EQ(16, buffer->width());
   EXPECT_EQ(16, buffer->height());
   // Try to trigger use-after-free errors by writing to y-plane.
-  memset(buffer->data(kYPlane), 0xA5, 16 * buffer->stride(kYPlane));
+  memset(buffer->MutableData(kYPlane), 0xA5, 16 * buffer->stride(kYPlane));
 }
 
 }  // namespace webrtc