Change SVGUseElement to buildPendingResource on attach

Previously, not calling buildPendingResouce caused SVGUseElement to not
update its shadow children if nodes were lazy attached. This change
causes SVGUseElement to rebuild its shadow tree on attach if needed
and changes invalidateShadowTree() to check attached() instead of
renderer().

BUG=279434

Review URL: https://chromiumcodereview.appspot.com/23614008

git-svn-id: svn://svn.chromium.org/blink/trunk@156885 bbb929c8-8fbe-4397-9dbb-9b2b20218538