Gradients should restrict possible content

LayoutTests/svg/custom/gradient-containing-disallow-elements.svg

Index: LayoutTests/svg/custom/gradient-containing-disallow-elements.svg
diff --git a/LayoutTests/svg/custom/gradient-containing-disallow-elements.svg b/LayoutTests/svg/custom/gradient-containing-disallow-elements.svg
new file mode 100644
index 0000000000000000000000000000000000000000..e715929aa93b1677c4eadf1b29bb2242137572e4
--- /dev/null
+++ b/LayoutTests/svg/custom/gradient-containing-disallow-elements.svg
@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg">
+  <title id="test-title">gradient containing disallowed elements</title>
+  <linearGradient id="grad1" gradientUnits="objectBoundingBox" x1="0" y1="0" x2="1" y2="0">
+    <stop stop-color="blue" offset="0"/>
+    <stop stop-color="red" offset="1"/>
+    <rect width="100" height="100" fill="red"/>
+    <g>
+      <foreignObject/>
+    </g>
+    <linearGradient id="grad2" gradientUnits="objectBoundingBox" x1="0" y1="0" x2="1" y2="0">
+      <stop stop-color="green" offset="0"/>
+      <stop stop-color="purple" offset="1"/>
+    </linearGradient>
+  </linearGradient>
+  <rect x="20" y="10" width="440" height="30" fill="url(#grad1)" />
+  <rect x="20" y="50" width="440" height="30" fill="url(#grad2)" />
+</svg>