Update LayoutTests by removing child elements of <shadow> element

Remove the children of <shadow> element from the layout tests.
Even though child elements are ignored, it can be confusing sometimes.

BUG=334957

Review URL: https://codereview.chromium.org/441223002

git-svn-id: svn://svn.chromium.org/blink/trunk@179694 bbb929c8-8fbe-4397-9dbb-9b2b20218538