RenderLayer::setHasVisibleContent shouldn't call dirtyZOrderLists()

There's no connection between being visible and z-order lists. There's no
reason to mark the z-order list as dirty when changing visibility status.

Review URL: https://codereview.chromium.org/320593004

git-svn-id: svn://svn.chromium.org/blink/trunk@175708 bbb929c8-8fbe-4397-9dbb-9b2b20218538