Don't explicitly check isRoot in layerTypeRequired.

The document element is always coerced to being a block, so
the check in RenderBoxModelObject is never hit.

The document element also always creates a stacking context,
so the isRoot check in RenderBox is redundant with the hasAutoZIndex
check.

Review URL: https://codereview.chromium.org/224733017

git-svn-id: svn://svn.chromium.org/blink/trunk@170987 bbb929c8-8fbe-4397-9dbb-9b2b20218538