Custom Elements: Encapsulates Async and Sync Queues into one class.

Source/core/core.gypi

 {
     'includes': [
       'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationEffect.idl',
             'animation/AnimationPlayer.idl',
@@ skipping 2163 matching lines @@
             'dom/custom/CustomElementDescriptorHash.h',
             'dom/custom/CustomElementException.cpp',
             'dom/custom/CustomElementException.h',
             'dom/custom/CustomElementLifecycleCallbacks.h',
             'dom/custom/CustomElementMicrotaskDispatcher.cpp',
             'dom/custom/CustomElementMicrotaskDispatcher.h',
             'dom/custom/CustomElementMicrotaskImportStep.cpp',
             'dom/custom/CustomElementMicrotaskImportStep.h',
             'dom/custom/CustomElementMicrotaskQueue.cpp',
             'dom/custom/CustomElementMicrotaskQueue.h',
+            'dom/custom/CustomElementMicrotaskQueueBase.cpp',
+            'dom/custom/CustomElementMicrotaskQueueBase.h',
             'dom/custom/CustomElementMicrotaskResolutionStep.cpp',
             'dom/custom/CustomElementMicrotaskResolutionStep.h',
             'dom/custom/CustomElementMicrotaskStep.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/CustomElementScheduler.cpp',
             'dom/custom/CustomElementScheduler.h',
+            'dom/custom/CustomElementSyncMicrotaskQueue.cpp',
+            'dom/custom/CustomElementSyncMicrotaskQueue.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 1210 matching lines @@
             'loader/MixedContentCheckerTest.cpp',
             'page/NetworkStateNotifierTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }