Remove rtc::Buffer::length(), since no one uses it anymore

Chromium now uses size() instead, just like WebRTC.

This CL also fixes a new length() call that had crept in.

R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/44119004

Cr-Commit-Position: refs/heads/master@{#9024}
2 files changed