Make Custom Element registration context creation explicit.

LayoutTests/fast/dom/custom/registration-context-isolation.html

Index: LayoutTests/fast/dom/custom/registration-context-isolation.html
diff --git a/LayoutTests/fast/dom/custom/registration-context-isolation.html b/LayoutTests/fast/dom/custom/registration-context-isolation.html
index 302a53f3516c7c70d18aaea422ad26754d71d25a..d82bb8fcb275a1cb27f1e5418c42907567708410 100644
--- a/LayoutTests/fast/dom/custom/registration-context-isolation.html
+++ b/LayoutTests/fast/dom/custom/registration-context-isolation.html
@@ -148,4 +148,41 @@ withFrame(t.step_func(function (frame) {
 
 })();
 
+(function () {
+
+var t = async_test('registration context should not be created by ' +
+                   'DOMImplementation-created documents');
+
+withFrame(t.step_func(function (frame) {
+    // Test transitively sloughing off a registration context through
+    // multiple createDocument/createHTMLDocument steps.
+
+    var documentA = frame.contentDocument;
+
+    // This document is not HTML, XHTML; it will not process custom elements.
+    var documentB = documentA.implementation.createDocument(null, '');
+    assert_throws(
+        'NOT_SUPPORTED_ERR',
+        function() { documentB.registerElement('x-a'); });
+
+    // This document will not process custom elements because there is
+    // nothing to inherit from B.
+    var documentC = documentB.implementation.createHTMLDocument();
+    assert_throws(
+        'NOT_SUPPORTED_ERR',
+        function() { documentC.registerElement('x-b'); });
+
+    // Nor this one.
+    var documentD = documentC.implementation.createDocument(
+        'http://www.w3.org/1999/xhtml', 'html');
+    assert_throws(
+        'NOT_SUPPORTED_ERR',
+        function() { documentD.registerElement('x-c'); });
+
+    frame.remove();
+    t.done();
+}));
+
+})();
+
 </script>