Add a stack of queues of elements with reaction queues.

third_party/WebKit/Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 2621 matching lines @@
             'dom/custom/CustomElement.cpp',
             'dom/custom/CustomElement.h',
             'dom/custom/CustomElementDefinition.cpp',
             'dom/custom/CustomElementDefinition.h',
             'dom/custom/CustomElementDefinitionBuilder.h',
             'dom/custom/CustomElementDescriptor.h',
             'dom/custom/CustomElementDescriptorHash.h',
             'dom/custom/CustomElementReaction.h',
             'dom/custom/CustomElementReactionQueue.cpp',
             'dom/custom/CustomElementReactionQueue.h',
+            'dom/custom/CustomElementReactionStack.cpp',
+            'dom/custom/CustomElementReactionStack.h',
             'dom/custom/CustomElementUpgradeSorter.cpp',
             'dom/custom/CustomElementUpgradeSorter.h',
             'dom/custom/CustomElementsRegistry.cpp',
             'dom/custom/CustomElementsRegistry.h',
             'dom/custom/V0CustomElement.cpp',
             'dom/custom/V0CustomElement.h',
             'dom/custom/V0CustomElementAsyncImportMicrotaskQueue.cpp',
             'dom/custom/V0CustomElementAsyncImportMicrotaskQueue.h',
             'dom/custom/V0CustomElementCallbackInvocation.cpp',
             'dom/custom/V0CustomElementCallbackInvocation.h',
@@ skipping 1326 matching lines @@
             'dom/NthIndexCacheTest.cpp',
             'dom/RangeTest.cpp',
             'dom/ScriptRunnerTest.cpp',
             'dom/SelectorQueryTest.cpp',
             'dom/StyleElementTest.cpp',
             'dom/StyleEngineTest.cpp',
             'dom/TextTest.cpp',
             'dom/TreeScopeStyleSheetCollectionTest.cpp',
             'dom/TreeScopeTest.cpp',
             'dom/URLSearchParamsTest.cpp',
-            'dom/custom/CustomElementTest.cpp',
             'dom/custom/CustomElementDescriptorTest.cpp',
             'dom/custom/CustomElementReactionQueueTest.cpp',
+            'dom/custom/CustomElementReactionStackTest.cpp',
+            'dom/custom/CustomElementReactionTestHelpers.h',
+            'dom/custom/CustomElementTest.cpp',
+            'dom/custom/CustomElementTestHelpers.h',
             'dom/custom/CustomElementUpgradeSorterTest.cpp',
             'dom/shadow/FlatTreeTraversalTest.cpp',
             'editing/EditingCommandTest.cpp',
             'editing/EditingStrategyTest.cpp',
             'editing/EditingTestBase.cpp',
             'editing/EditingTestBase.h',
             'editing/EditingUtilitiesTest.cpp',
             'editing/EditorTest.cpp',
             'editing/FrameSelectionTest.cpp',
             'editing/GranularityStrategyTest.cpp',
@@ skipping 139 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }