HTML Imports: No more BlockingDocumentCreation.

Source/core/core.gypi

 {
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/Animation.idl',
             'animation/AnimationPlayer.idl',
             'animation/TimedItem.idl',
             'animation/Timeline.idl',
             'animation/Timing.idl',
             'clipboard/DataTransfer.idl',
@@ skipping 2153 matching lines @@
             '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/CustomElementUpgradeCandidateMap.cpp',
             'dom/custom/CustomElementUpgradeCandidateMap.h',
+            'dom/custom/RefCountedCustomElementMicrotaskQueue.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',
             'dom/shadow/SelectRuleFeatureSet.cpp',
             'dom/shadow/SelectRuleFeatureSet.h',
@@ skipping 1198 matching lines @@
             'html/track/vtt/BufferedLineReaderTest.cpp',
             'html/track/vtt/VTTScannerTest.cpp',
             'rendering/RenderOverflowTest.cpp',
             'rendering/shapes/BoxShapeTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }