Simplify VectorMover<true, T>::swap()

Simplify VectorMover<true, T>::swap() - use std::swap_ranges instead
of creating the loop ourselves, plus the patch removes a "FIXME"
comment - compiler already produces the most efficient code for POD
types swap, so there is no need to introduce compex algorithms for
that (checked with GCC 4.7.2).

Review URL: https://codereview.chromium.org/208253003

git-svn-id: svn://svn.chromium.org/blink/trunk@169747 bbb929c8-8fbe-4397-9dbb-9b2b20218538