Make WebFilterOperations pure virtual

This turns WebFilterOperations into a pure virtual interface.

This depends on chromium revision http://crrev.com/208531.

BUG=181613

Review URL: https://chromiumcodereview.appspot.com/17286006

git-svn-id: svn://svn.chromium.org/blink/trunk@153095 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use toDocument instead of static_cast<Document*>.

BUG=

Review URL: https://chromiumcodereview.appspot.com/17885002

git-svn-id: svn://svn.chromium.org/blink/trunk@153096 bbb929c8-8fbe-4397-9dbb-9b2b20218538