Run Mutation Observer and Custom Element callbacks consistently at microtask checkpoint

Source/core/html/parser/HTMLScriptRunner.cpp

Index: Source/core/html/parser/HTMLScriptRunner.cpp
diff --git a/Source/core/html/parser/HTMLScriptRunner.cpp b/Source/core/html/parser/HTMLScriptRunner.cpp
index 93514ef1a44e33c9d93079f4d0dd316b6406865c..a1f1f6ccf831a4d08c6c577b1a289bdafe6e1e07 100644
--- a/Source/core/html/parser/HTMLScriptRunner.cpp
+++ b/Source/core/html/parser/HTMLScriptRunner.cpp
@@ -29,11 +29,10 @@
 #include "HTMLNames.h"
 #include "bindings/v8/ScriptSourceCode.h"
 #include "core/dom/Attribute.h"
-#include "core/dom/CustomElementRegistry.h"
 #include "core/dom/Element.h"
 #include "core/dom/Event.h"
 #include "core/dom/IgnoreDestructiveWriteCountIncrementer.h"
-#include "core/dom/MutationObserver.h"
+#include "core/dom/Microtask.h"
 #include "core/dom/ScriptElement.h"
 #include "core/html/parser/HTMLInputStream.h"
 #include "core/html/parser/HTMLScriptRunnerHost.h"
@@ -130,8 +129,7 @@ void HTMLScriptRunner::executePendingScriptAndDispatchEvent(PendingScript& pendi
         stopWatchingForLoad(pendingScript);
 
     if (!isExecutingScript()) {
-        CustomElementRegistry::deliverAllLifecycleCallbacks();
-        MutationObserver::deliverAllMutations();
+        Microtask::performCheckpoint();
     }
 
     // Clear the pending script before possible rentrancy from executeScript()
@@ -296,8 +294,7 @@ void HTMLScriptRunner::runScript(Element* script, const TextPosition& scriptStar
         // unfortuantely no obvious way to tell if prepareScript is going to
         // execute the script from out here.
         if (!isExecutingScript()) {
-            CustomElementRegistry::deliverAllLifecycleCallbacks();
-            MutationObserver::deliverAllMutations();
+            Microtask::performCheckpoint();
         }
 
         InsertionPointRecord insertionPointRecord(m_host->inputStream());