rtc::Buffer: Restore length method for backwards compatibility
rtc::Buffer: Restore length method for backwards compatibility

TBR=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43939004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8845}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 4d14592c6744d12c24968b4fc0b5a3bdae3d1093

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc