Trivial clean ups to DragState.h

- remove unnecessary includes (the forward declarations are enough).

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/48423003

git-svn-id: svn://svn.chromium.org/blink/trunk@161236 bbb929c8-8fbe-4397-9dbb-9b2b20218538