Remove children of <shadow> element added by JS

Update the Layout tests for shadow so that they dont use child
elements of <shadow>. In this patch, childs which are added through
JS is being removed.

BUG=334957

Review URL: https://codereview.chromium.org/450533007

git-svn-id: svn://svn.chromium.org/blink/trunk@179908 bbb929c8-8fbe-4397-9dbb-9b2b20218538