Revert 170996 "Speculative revert 170987 "Don't explicitly check..."

> Speculative revert 170987 "Don't explicitly check isRoot in layerTypeRequired."
> 
> http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win7/builds/14731
> 
> > 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
> 
> TBR=ojan@chromium.org
> 
> Review URL: https://codereview.chromium.org/227043013

TBR=jianli@chromium.org

Review URL: https://codereview.chromium.org/227913002

git-svn-id: svn://svn.chromium.org/blink/trunk@170998 bbb929c8-8fbe-4397-9dbb-9b2b20218538