rtc::Buffer: Accept void* in addition to the byte-sized types

We used to accept void* (until 9478437f), and we'll have to continue
to do so for a little while longer, until Chromium doesn't need it
anymore.

TBR=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48179004

Cr-Commit-Position: refs/heads/master@{#9035}