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

Source/core/dom/custom/CustomElementMicrotaskResolutionStep.cpp

Index: Source/core/dom/custom/CustomElementMicrotaskResolutionStep.cpp
diff --git a/Source/core/dom/custom/CustomElementMicrotaskResolutionStep.cpp b/Source/core/dom/custom/CustomElementMicrotaskResolutionStep.cpp
index e5edad0115e857da48882842f578346d613dd6d2..26ecc00a676f72baf6b9a4c11b9886425660e072 100644
--- a/Source/core/dom/custom/CustomElementMicrotaskResolutionStep.cpp
+++ b/Source/core/dom/custom/CustomElementMicrotaskResolutionStep.cpp
@@ -58,4 +58,16 @@ CustomElementMicrotaskStep::Result CustomElementMicrotaskResolutionStep::process
     return CustomElementMicrotaskStep::Continue;
 }
 
+#if !defined(NDEBUG)
+
+void CustomElementMicrotaskResolutionStep::show(unsigned indent)
+{
+    for (unsigned i = 0; i < indent; ++i)
+        fprintf(stderr, " ");
+    fprintf(stderr, "Resolution: ");
+    m_element->outerHTML().show();
+}
+
+#endif
+
 } // namespace WebCore