Refactoring: Let ChannelPosix adopt ScopedFD.

This gets rid of raw close() call usign base::ScopedFD.
Ownership of FDs become clearer.

This is a preparation for kiling base::FileDescriptor.

R=jam@chromium.org, agl@chromium.org
BUG=415294

Review URL: https://codereview.chromium.org/602193004

Cr-Commit-Position: refs/heads/master@{#297285}