After r47630, fast/repaint/background-generated.html is failing because the unspecified background-size is interpreted as 'auto' by mustRepaintFillLayers(). The type is initialized to SizeLength but it should be SizeNone.
Created attachment 38533 [details]
Created attachment 38553 [details]
Comment on attachment 38553 [details]
Fixed in <http://trac.webkit.org/projects/webkit/changeset/47749>.
Thanks for closing the bug, Eric!