Trivial data() and mutable_data() implementations
Trivial data() and mutable_data() implementations

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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17532}

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