Add support for "image-set"

third_party/WebKit/LayoutTests/cssom/cssvalue-comparison.html

Index: third_party/WebKit/LayoutTests/cssom/cssvalue-comparison.html
diff --git a/third_party/WebKit/LayoutTests/cssom/cssvalue-comparison.html b/third_party/WebKit/LayoutTests/cssom/cssvalue-comparison.html
index 5e0feb9e0584721c809204704fdf332972856094..0414a5335d10ada4e71b56c81d7bdde9018cb3aa 100644
--- a/third_party/WebKit/LayoutTests/cssom/cssvalue-comparison.html
+++ b/third_party/WebKit/LayoutTests/cssom/cssvalue-comparison.html
@@ -65,6 +65,7 @@ function run() {
                   {"-webkit-transform" : ["rotate(30deg)", "translate(50px,50px)", "scale(2,4)", "skew(30deg,20deg)", "matrix(0.4,0.5,-0.5,0.4,0,0)"]}, // transforms
                   {"-webkit-line-box-contain" : ["inline-box", "font", "glyphs", "replaced"]}, // line-box-contain
                   {"background-image" : ["-webkit-image-set(url(dummy://test.png) 1x, url(dummy://test.png) 2x)", "-webkit-image-set(url(dummy://small.png) 2x, url(dummy://big.png) 3x)"]}, // image set
+                  {"background-image" : ["image-set(url(dummy://test.png) 1x, url(dummy://test.png) 2x)", "image-set(url(dummy://small.png) 2x, url(dummy://big.png) 3x)"]}, // image set
                   {"-webkit-filter" : ["grayscale(100%) sepia(100%)", "sepia(10%) grayscale(50%)"]}, // filter
                   {"-webkit-dashboard-region" : ["dashboard-region(label circle)", "dashboard-region(label circle 1px 2px 3px 4px) dashboard-region(label rectangle 5px 6px 7px 8px)"]} // dashboard region
                 ];