Minor clean ups to BackForwardClient.h

- Include only the necessary header.
- Forward declare HistoryItem.

R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19518007

git-svn-id: svn://svn.chromium.org/blink/trunk@154619 bbb929c8-8fbe-4397-9dbb-9b2b20218538