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

Source/core/dom/custom/CustomElementMicrotaskStep.h

Index: Source/core/dom/custom/CustomElementMicrotaskStep.h
diff --git a/Source/core/dom/custom/CustomElementMicrotaskStep.h b/Source/core/dom/custom/CustomElementMicrotaskStep.h
index 625067ec1e327af11ee00fcfa51ad341fa0f6db1..ba04d2968538cccc2a435adf607e6e2a29721c8a 100644
--- a/Source/core/dom/custom/CustomElementMicrotaskStep.h
+++ b/Source/core/dom/custom/CustomElementMicrotaskStep.h
@@ -42,11 +42,16 @@ public:
     virtual ~CustomElementMicrotaskStep() { }
 
     enum Result {
-        Continue   = 1 << 0,
-        ShouldStop = 1 << 1
+        Continue     = 1 << 0,
+        ShouldStop   = 1 << 1,
+        ShouldRemain = 1 << 2
     };
 
     virtual Result process() = 0;
+
+#if !defined(NDEBUG)
+    virtual void show(unsigned indent) { }
+#endif
 };
 
 }