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

git-svn-id: svn://svn.chromium.org/blink/trunk@170996 bbb929c8-8fbe-4397-9dbb-9b2b20218538