Store marker identifiers as AtomicString

Marker identifiers can be better stored as AtomicString (like for instance clip-path identifiers) since
they wil llikely share the same value as an id attribute.

Review URL: https://codereview.chromium.org/203723007

git-svn-id: svn://svn.chromium.org/blink/trunk@169556 bbb929c8-8fbe-4397-9dbb-9b2b20218538