Add a custom element to own structure of shadow DOM plugin placeholders.

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 2799 matching lines @@
             'html/shadow/MediaControlElementTypes.cpp',
             'html/shadow/MediaControlElementTypes.h',
             'html/shadow/MediaControlElements.cpp',
             'html/shadow/MediaControlElements.h',
             'html/shadow/MediaControls.cpp',
             'html/shadow/MediaControls.h',
             'html/shadow/MeterShadowElement.cpp',
             'html/shadow/MeterShadowElement.h',
             'html/shadow/PickerIndicatorElement.cpp',
             'html/shadow/PickerIndicatorElement.h',
+            'html/shadow/PluginPlaceholderElement.cpp',
+            'html/shadow/PluginPlaceholderElement.h',
             'html/shadow/ProgressShadowElement.cpp',
             'html/shadow/ProgressShadowElement.h',
             'html/shadow/ShadowElementNames.cpp',
             'html/shadow/ShadowElementNames.h',
             'html/shadow/SliderThumbElement.cpp',
             'html/shadow/SliderThumbElement.h',
             'html/shadow/SpinButtonElement.cpp',
             'html/shadow/SpinButtonElement.h',
             'html/shadow/TextControlInnerElements.cpp',
             'html/shadow/TextControlInnerElements.h',
@@ skipping 669 matching lines @@
             'rendering/shapes/BoxShapeTest.cpp',
             'rendering/style/OutlineValueTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'streams/ReadableStreamTest.cpp',
             'testing/UnitTestHelpers.cpp',
             'testing/UnitTestHelpers.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }