This will allow migrating existing references to data_ over to data() and mutable_data(), so that https://codereview.webrtc.org/2750783004/ can be imported cleanly. BUG=webrtc:7343 TBR=henrika@webrtc.org Review-Url: https://codereview.webrtc.org/2795103002 Cr-Commit-Position: refs/heads/master@{#17532}