[SVG] Fix root element length values handling.

LayoutTests/svg/custom/svg-length-value-crash.svg

Index: LayoutTests/svg/custom/svg-length-value-crash.svg
diff --git a/LayoutTests/svg/custom/svg-length-value-crash.svg b/LayoutTests/svg/custom/svg-length-value-crash.svg
new file mode 100644
index 0000000000000000000000000000000000000000..c6302cac723e2a217427bec9a6cca274f73df50c
--- /dev/null
+++ b/LayoutTests/svg/custom/svg-length-value-crash.svg
@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg">
+  <!-- Test for http://crbugs.com/317484 -->
+  <rect width="100" height="100" fill="green"/>
+  <text y="230">PASS: did not crash.</text>
+  <script>
+    if (window.testRunner)
+      testRunner.dumpAsText();
+
+    var w = document.rootElement.width.baseVal.value;
+    var h = document.rootElement.height.baseVal.value;
+  </script>
+</svg>