Don't allow <stop> as layout tree child of <svg> or <g>

LayoutTests/svg/dom/content-model.html

Index: LayoutTests/svg/dom/content-model.html
diff --git a/LayoutTests/svg/dom/content-model.html b/LayoutTests/svg/dom/content-model.html
index f463dd81c24828c6f5244887010d50588bf7c529..ee6cce3211955dcdc55a9b884f3c3acb6b4f4884 100644
--- a/LayoutTests/svg/dom/content-model.html
+++ b/LayoutTests/svg/dom/content-model.html
@@ -101,6 +101,28 @@
         // http://dev.w3.org/fxtf/filters/#FilterElement
         'CLASS_DESCRIPTIVE', 'CLASS_FILTER_PRIMITIVE', 'animate', 'set'
       ]);
+      validateTag('linearGradient', [
+        // https://svgwg.org/svg2-draft/pservers.html#LinearGradientElement
+        'CLASS_DESCRIPTIVE', 'CLASS_PAINT_SERVER', 'animate', 'animateTransform', 'script', 'set',
+        'stop'
+      ]);
+      validateTag('g', [
+        // https://svgwg.org/svg2-draft/struct.html#GElement
+        'CLASS_ANIMATION', 'CLASS_DESCRIPTIVE', 'CLASS_PAINT_SERVER', 'CLASS_SHAPE',
+        'CLASS_STRUCTURAL', 'a', 'clipPath', 'cursor', 'filter', 'foreignObject', 'image', 'marker',
+        'mask', 'script', 'style', 'switch', 'text', 'view'
+      ]);
+      validateTag('radialGradient', [
+        // https://svgwg.org/svg2-draft/pservers.html#RadialGradientElement
+        'CLASS_DESCRIPTIVE', 'CLASS_PAINT_SERVER', 'animate', 'animateTransform', 'script', 'set',
+        'stop'
+      ]);
+      validateTag('svg', [
+        // https://svgwg.org/svg2-draft/struct.html#SVGElement
+        'CLASS_ANIMATION', 'CLASS_DESCRIPTIVE', 'CLASS_PAINT_SERVER', 'CLASS_SHAPE',
+        'CLASS_STRUCTURAL', 'a', 'clipPath', 'cursor', 'filter', 'foreignObject', 'image', 'marker',
+        'mask', 'script', 'style', 'switch', 'text', 'view'
+      ]);
     </script>
   </body>
 </html>