VideoFrameBuffer: Make non-const data access explicit

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 625160be1199501860db0f096cdd92bab57af05c..a1596ebb09d6f3846fae093474bebc523ba16d8c 100644
--- a/webrtc/common_video/i420_buffer_pool_unittest.cc
+++ b/webrtc/common_video/i420_buffer_pool_unittest.cc
@@ -68,7 +68,7 @@ TEST(TestI420BufferPool, FrameValidAfterPoolDestruction) {
   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