Let unresolved custom element go through CustomElementCallbackQueue.

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'animation/ElementAnimation.idl',
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSKeyframeRule.idl',
@@ skipping 2020 matching lines @@
             'dom/custom/CustomElementCallbackScheduler.h',
             'dom/custom/CustomElementDefinition.cpp',
             'dom/custom/CustomElementDefinition.h',
             'dom/custom/CustomElementDescriptor.h',
             'dom/custom/CustomElementDescriptorHash.h',
             'dom/custom/CustomElementException.cpp',
             'dom/custom/CustomElementException.h',
             'dom/custom/CustomElementLifecycleCallbacks.h',
             'dom/custom/CustomElementObserver.cpp',
             'dom/custom/CustomElementObserver.h',
+            'dom/custom/CustomElementProcessingStep.h',
             'dom/custom/CustomElementRegistrationContext.cpp',
             'dom/custom/CustomElementRegistrationContext.h',
             'dom/custom/CustomElementRegistry.cpp',
             'dom/custom/CustomElementRegistry.h',
+            'dom/custom/CustomElementResolutionStep.cpp',
+            'dom/custom/CustomElementResolutionStep.h',
             'dom/custom/CustomElementUpgradeCandidateMap.cpp',
             'dom/custom/CustomElementUpgradeCandidateMap.h',
             'dom/shadow/ComposedTreeWalker.cpp',
             'dom/shadow/ComposedTreeWalker.h',
             'dom/shadow/ContentDistribution.cpp',
             'dom/shadow/ContentDistribution.h',
             'dom/shadow/ElementShadow.cpp',
             'dom/shadow/ElementShadow.h',
             'dom/shadow/InsertionPoint.cpp',
             'dom/shadow/InsertionPoint.h',
@@ skipping 1213 matching lines @@
             'platform/animation/TimingFunctionTestHelper.h',
             'platform/animation/TimingFunctionTestHelper.cpp',
             'platform/animation/TimingFunctionTestHelperTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.h',
             'testing/UnitTestHelpers.cpp',
         ],
     }
 }