Remove useless null checks from Node::document() call sites

Remove useless null checks from Node::document() call sites now that all nodes
have an associated Document at all times. This is a preparation patch to later
have Node::document() return a reference.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23717008

git-svn-id: svn://svn.chromium.org/blink/trunk@156986 bbb929c8-8fbe-4397-9dbb-9b2b20218538