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

Source/core/html/imports/HTMLImportChild.cpp

Index: Source/core/html/imports/HTMLImportChild.cpp
diff --git a/Source/core/html/imports/HTMLImportChild.cpp b/Source/core/html/imports/HTMLImportChild.cpp
index 4309e071e08606f285a5ed65c40f950a1ef3f899..33bf0097961d01b6977ecaa406871d989ad9de76 100644
--- a/Source/core/html/imports/HTMLImportChild.cpp
+++ b/Source/core/html/imports/HTMLImportChild.cpp
@@ -62,11 +62,8 @@ HTMLImportChild::~HTMLImportChild()
     // importDestroyed() should be called before the destruction.
     ASSERT(!m_loader);
 
-    if (m_customElementMicrotaskStep) {
-        // if Custom Elements were blocked, must unblock them before death
-        m_customElementMicrotaskStep->importDidFinish();
-        m_customElementMicrotaskStep = 0;
-    }
+    if (m_customElementMicrotaskStep)
+        m_customElementMicrotaskStep->importWasDestroyed();
 
     if (m_client)
         m_client->importChildWasDestroyed(this);
@@ -99,18 +96,25 @@ void HTMLImportChild::didFinish()
     if (m_client)
         m_client->didFinish();
 
-    if (m_customElementMicrotaskStep) {
-        m_customElementMicrotaskStep->importDidFinish();
-        m_customElementMicrotaskStep = 0;
-    }
+    ASSERT(!m_customElementMicrotaskStep);
 }
 
 void HTMLImportChild::didFinishLoading()
 {
+    // This triggers closing handshake between CustomElementMicrotaskImportStep.
+    // It eventually tells back through didFinishMicrotask(), then this import becomes "done".
+    if (m_customElementMicrotaskStep)
+        m_customElementMicrotaskStep->importDidLoad(this);
     clearResource();
     stateWillChange();
 }
 
+void HTMLImportChild::didFinishMicrotask()
+{
+    m_customElementMicrotaskStep = 0;
+    stateWillChange();
+}
+
 Document* HTMLImportChild::importedDocument() const
 {
     if (!m_loader)
@@ -157,7 +161,7 @@ void HTMLImportChild::ensureLoader()
     else
         createLoader();
 
-    if (isSync() && !isDone()) {
+    if (!isDone() && !formsCycle()) {
         ASSERT(!m_customElementMicrotaskStep);
         m_customElementMicrotaskStep = CustomElement::didCreateImport(this);
     }
@@ -181,7 +185,7 @@ void HTMLImportChild::shareLoader(HTMLImportChild* loader)
 
 bool HTMLImportChild::isDone() const
 {
-    return m_loader && m_loader->isDone();
+    return m_loader && m_loader->isDone() && !m_customElementMicrotaskStep;
 }
 
 bool HTMLImportChild::loaderHasError() const