Return an infinite rect when calling RenderLayerClipper::parentClipRects on the root context layer

When calling RenderLayerClipper::backgroundClipRect on the
RenderLayerClipper object corresponding to the RenderLayer
which is equal to clipRectsContext.rootLayer, it first calls
parentClipRects on the clipper of that RenderLayer's parent.
In turn, this starts a recursive walk up the RenderLayer
tree in updateClipRects. This is incorrect, since a
RenderLayerClipper object is unable to clip above rootLayer
in the render tree. In particular, the recursion condition
in updateClipRects
depends on the recursion starting at a layer which is below
or equal to rootLayer.

BUG=336563

Review URL: https://codereview.chromium.org/211683003

git-svn-id: svn://svn.chromium.org/blink/trunk@170234 bbb929c8-8fbe-4397-9dbb-9b2b20218538