Add seek methods to FileWrapper, and refactor WavReader to use that class instead. Bug: webrtc:6463 Change-Id: Ifbb1989a072da6280ea5fc04b4beff991614dd53 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147265 Commit-Queue: Niels Moller <nisse@webrtc.org> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org> Cr-Commit-Position: refs/heads/master@{#28770}