REGRESSION(r171966): Custom elements in async imports don't get upgrade.

Source/core/html/imports/HTMLImport.cpp

Index: Source/core/html/imports/HTMLImport.cpp
diff --git a/Source/core/html/imports/HTMLImport.cpp b/Source/core/html/imports/HTMLImport.cpp
index 96a23cca9f211e265dfb905177b61f454a54a5aa..4726f7132355c1c9006e6bc4d2adb695e1ec7cc4 100644
--- a/Source/core/html/imports/HTMLImport.cpp
+++ b/Source/core/html/imports/HTMLImport.cpp
@@ -54,6 +54,17 @@ bool HTMLImport::precedes(HTMLImport* import)
     return false;
 }
 
+bool HTMLImport::formsCycle() const
+{
+    for (const HTMLImport* i = this->parent(); i; i = i->parent()) {
+        if (i->document() == this->document())
+            return true;
+    }
+
+    return false;
+
+}
+
 void HTMLImport::appendImport(HTMLImport* child)
 {
     appendChild(child);