Replace 'static AtomicString ...' with DEFINE_STATIC_LOCAL.

Tiny optimiziation, should have zero visible effect on the web, other
than making process shutdown a very very very tiny bit faster.

Review URL: https://codereview.chromium.org/26538003

git-svn-id: svn://svn.chromium.org/blink/trunk@159567 bbb929c8-8fbe-4397-9dbb-9b2b20218538