Refactor fileutils.cc/h and fileutils_unittests.cc into their own targets.

This will allow for custom implementations downstream.

R=kjellander@webrtc.org, phoglund@webrtc.org
BUG=webrtc:6727

Review-Url: https://codereview.webrtc.org/2548713003
Cr-Original-Commit-Position: refs/heads/master@{#15423}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 37535bfb7fcb46df9382e6976ce9ac99c3fb1c3e
3 files changed