[css-ui-3] Mark support for non fixed size svg as optional (#7430)

This is to match the following spec change: w3c/csswg-drafts@5dcad30

css/css-ui-3/cursor-image-005-nfs.html

@@ -4,7 +4,7 @@
 <link rel="reviewer" title="Florian Rivoal" href="http://florian.rivoal.net/"> <!-- 2015-04-12 -->
 <link rel="help" href="http://www.w3.org/TR/css3-ui/#cursor">
 <link rel="help" href="http://www.w3.org/TR/css3-images/#default-object-size">
-<meta name="flags" content="interact svg">
+<meta name="flags" content="interact svg may">
 <meta charset="UTF-8">
 <meta name="assert" content="Test checks that an SVG image with no fixed size is supported as a custom cursor at the default object size for cursor images.">
 <style>