Split MoveReadPosition into Forward and Backward versions.

This makes the interface consistent in its use of size_t, and will
reduce casts in callers as they shift to size_t, e.g. this CL:
https://codereview.webrtc.org/1227203003/

Review URL: https://codereview.webrtc.org/1252943007

Cr-Commit-Position: refs/heads/master@{#9646}
4 files changed