Remove UnwrapTraits

Remove UnwrapTraits::ForwardType, which is no longer used, and extract
UnwrapTraits::Unwrap to be non member function.

BUG=554299

Review URL: https://codereview.chromium.org/1698223002

Cr-Commit-Position: refs/heads/master@{#375576}
3 files changed