Don't call ElementShadow::populate() recursively.

Since the node in the pool is never be an active insertion point, we don't have to call populate() again to the node in the pool.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22929030

git-svn-id: svn://svn.chromium.org/blink/trunk@156617 bbb929c8-8fbe-4397-9dbb-9b2b20218538