Scale factor values in general can go 0 or even to negative, but device scale factor is about device pixel/CSS pixel ratio. It makes hardly any sense to have that as 0 or negative.
Created attachment 227892 [details] Patch
Comment on attachment 227892 [details] Patch r=me
<rdar://problem/16293570>
Committed r166323: <http://trac.webkit.org/changeset/166323>