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-Original-Commit-Position: refs/heads/master@{#9646}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d40af69278f867965fd10313f9c99f90266a9469
4 files changed