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

Source/core/html/imports/HTMLImportLoader.cpp

Index: Source/core/html/imports/HTMLImportLoader.cpp
diff --git a/Source/core/html/imports/HTMLImportLoader.cpp b/Source/core/html/imports/HTMLImportLoader.cpp
index 7bf0a30622794c5a9973efe77303ca3b74ecd7dc..fa70f24f665bd997a441355b68ce5d1832c9daad 100644
--- a/Source/core/html/imports/HTMLImportLoader.cpp
+++ b/Source/core/html/imports/HTMLImportLoader.cpp
@@ -46,7 +46,6 @@ namespace WebCore {
 HTMLImportLoader::HTMLImportLoader(HTMLImportsController* controller)
     : m_controller(controller)
     , m_state(StateLoading)
-    , m_microtaskQueue(CustomElementMicrotaskQueue::create())
 {
 }
 
@@ -223,4 +222,11 @@ PassRefPtr<CustomElementMicrotaskQueue> HTMLImportLoader::microtaskQueue() const
     return m_microtaskQueue;
 }
 
+PassRefPtr<CustomElementMicrotaskQueue> HTMLImportLoader::ensureMicrotaskQueue()
+{
+    if (!m_microtaskQueue)
+        m_microtaskQueue = CustomElementMicrotaskQueue::create();
+    return m_microtaskQueue;
+}
+
 } // namespace WebCore