Add support for "image-set"

third_party/WebKit/LayoutTests/fast/css/script-tests/image-set-setting.js

 description("Test the setting of the -webkit-image-set function.");
 
 function testComputedStyle(property, fullRule)
 {
     var div = document.createElement("div");
     document.body.appendChild(div);
     div.setAttribute("style", property + ": " + fullRule);
     var computedValue = div.style.backgroundImage;
     document.body.removeChild(div);
     return computedValue;
 }
 
 function testImageSetRule(description, rule, expected)
 {
     debug("");
     debug(description + " : " + rule);
 
-    var fullRule = '-webkit-image-set(' + rule + ')';
+    var fullRule = 'image-set(' + rule + ')';
     var call = 'testComputedStyle(`background-image`, `' + fullRule + '`)';
     shouldBeEqualToString(call, fullRule);
 }
 
 testImageSetRule("Single value for background-image",
                 'url("http://www.webkit.org/a") 1x');
 
 testImageSetRule("Multiple values for background-image",
                 'url("http://www.webkit.org/a") 1x, url("http://www.webkit.org/b") 2x');
 
 testImageSetRule("Multiple values for background-image, out of order",
                 'url("http://www.webkit.org/c") 3x, url("http://www.webkit.org/b") 2x, url("http://www.webkit.org/a") 1x');
 
 testImageSetRule("Duplicate values for background-image",
                 'url("http://www.webkit.org/c") 1x, url("http://www.webkit.org/b") 2x, url("http://www.webkit.org/a") 1x');
 
 testImageSetRule("Fractional values for background-image",
                 'url("http://www.webkit.org/c") 0.2x, url("http://www.webkit.org/b") 2.3x, url("http://www.webkit.org/a") 12.3456x');
 
 // FIXME: We should also be testing the behavior of negative values somewhere, but it's currently
 // broken.  http://wkb.ug/100132
 
 successfullyParsed = true;