Only allocate a foreground layer for a CLM if there are composited negative-z-index-children.

Otherwise, those children can be rendered as pat of the software rendering of the current
layer and its non-compositing descendants.

BUG=

Review URL: https://codereview.chromium.org/535553002

git-svn-id: svn://svn.chromium.org/blink/trunk@181424 bbb929c8-8fbe-4397-9dbb-9b2b20218538