Add style checker exception for std::swap.

The style checker tries to catch std::x references and recommends
'using namespace std;' instead. However, 'using std::swap;' is an
established idiom for swapping objects in generic code, to permit ADL.

BUG=none

Review URL: https://codereview.chromium.org/276513005

git-svn-id: svn://svn.chromium.org/blink/trunk@173594 bbb929c8-8fbe-4397-9dbb-9b2b20218538