base::Bind implementation uses the function overload resolution with ADL
to dispatch bound parameters to Unwrap function. However, this works only
when the bound parameter is passed as a const reference. And other form
of bound parameters, such as rvalue-reference, are fallback to default
Unwrap and fails to compile.

This CL introduces UnwrapTraits<> as a traits struct, and converts Unwap()
function to UnwrapTraits<>::Unwrap(), so that the unwrapping function is
looked up by the normalized type name.

BUG=554299

Review-Url: https://codereview.chromium.org/2250373002
Cr-Commit-Position: refs/heads/master@{#415008}