Make Custom Elements V1 work in HTML imports documents

third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.cpp

Index: third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.cpp
diff --git a/third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.cpp b/third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.cpp
index 72245da7b33fedbfa0c30e88044881ded386ef5e..5aab3b88c118fd1ea36fc9d6642a7a0028260599 100644
--- a/third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.cpp
+++ b/third_party/WebKit/Source/core/dom/custom/CustomElementDefinition.cpp
@@ -52,8 +52,10 @@ static String errorMessageForConstructorResult(Element* element,
     if (element->parentNode())
         return "The result must not have a parent";
     // 6.1.7. If result's node document is not document, then throw a NotSupportedError.
-    if (&element->document() != &document)
+    if (&element->document() != &document
+        && element->document().contextDocument() != document.contextDocument()) {

[dominicc (has gone to gerrit), 2016/07/13 06:12:30]

I don't understand why we need this here. If element->document is document, then
element->document->foo is document->foo unless there's some subtle side effects
happening right at this point.

It would be good to write some C++ unit tests for this that demonstrates bailing
out at each point due to each case.

[kojii, 2016/07/13 06:47:16]

We'd like to check if contextDocument() match, so this could be just:
  if (element->document().contextDocument() != document.contextDocument())
but I left the original "&element->document() != &document" because it's the
common, fast path.

Should I just remove the fast path? It might be easier to read then.
One of JS test hit this. Can't that do? This is how I found the following spec
issue:
https://github.com/w3c/webcomponents/issues/533

         return "The result must be in the same document";
+    }
     // 6.1.8. If result's namespace is not the HTML namespace, then throw a NotSupportedError.
     if (element->namespaceURI() != HTMLNames::xhtmlNamespaceURI)
         return "The result must have HTML namespace";