Add a scoped context object for synchronous CE creation

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/AnimationEffectReadOnly.idl',
             'animation/AnimationEffectTiming.idl',
@@ skipping 2695 matching lines @@
             'dom/custom/CEReactionsScope.cpp',
             'dom/custom/CEReactionsScope.h',
             'dom/custom/CustomElement.cpp',
             'dom/custom/CustomElement.h',
             'dom/custom/CustomElementAdoptedCallbackReaction.cpp',
             'dom/custom/CustomElementAdoptedCallbackReaction.h',
             'dom/custom/CustomElementAttributeChangedCallbackReaction.cpp',
             'dom/custom/CustomElementAttributeChangedCallbackReaction.h',
             'dom/custom/CustomElementConnectedCallbackReaction.cpp',
             'dom/custom/CustomElementConnectedCallbackReaction.h',
+            'dom/custom/CustomElementCreationScope.h',
             'dom/custom/CustomElementDefinition.cpp',
             'dom/custom/CustomElementDefinition.h',
             'dom/custom/CustomElementDefinitionBuilder.h',
             'dom/custom/CustomElementDescriptor.h',
             'dom/custom/CustomElementDescriptorHash.h',
             'dom/custom/CustomElementDisconnectedCallbackReaction.cpp',
             'dom/custom/CustomElementDisconnectedCallbackReaction.h',
             'dom/custom/CustomElementReaction.cpp',
             'dom/custom/CustomElementReaction.h',
             'dom/custom/CustomElementReactionQueue.cpp',
@@ skipping 1574 matching lines @@
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/DedicatedWorkerTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }