Let some member functions of TreeScope take a const reference.

This change makes TreeScope::isInclusiveAncestorOf() and
TreeScope::comparePosition() take a const reference to a TreeScope instead of
a pointer.

This is a follow-up of r157111.

BUG=none
R=hayato@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23458019

git-svn-id: svn://svn.chromium.org/blink/trunk@157209 bbb929c8-8fbe-4397-9dbb-9b2b20218538