Add OVERRIDE to ui::TreeModelObserver overridden methods.

BUG=None
TEST=None

R=sky@chromium.org

Review URL: http://codereview.chromium.org/7046093

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@88827 0039d316-1c4b-4281-b951-d872f2087c98
void TreeView::AddObserverToModel() {
  if (model_ && !observer_added_) {
    model_->AddObserver(this);
    observer_added_ = true;
  }
}

void TreeView::AddObserverToModel() {
  if (model_ && !observer_added_) {
    model_->AddObserver(this);
    observer_added_ = true;
  }
}
