Factor out NullExecutionContext from unittest files.

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 3328 matching lines @@
             'testing/InternalSettings.cpp',
             'testing/InternalSettings.h',
             'testing/Internals.cpp',
             'testing/Internals.h',
             'testing/LayerRect.h',
             'testing/LayerRectList.cpp',
             'testing/LayerRectList.h',
             'testing/MallocStatistics.h',
             'testing/MockPagePopupDriver.cpp',
             'testing/MockPagePopupDriver.h',
+            'testing/NullExecutionContext.cpp',
+            'testing/NullExecutionContext.h',
             'testing/TypeConversions.h',
             'testing/v8/WebCoreTestSupport.cpp',
             'testing/v8/WebCoreTestSupport.h',
         ],
         'core_unittest_files': [
             'animation/AnimatableColorTest.cpp',
             'animation/AnimatableDoubleTest.cpp',
             'animation/AnimatableLengthTest.cpp',
             'animation/AnimatableNeutralTest.cpp',
             'animation/AnimatableStrokeDasharrayListTest.cpp',
@@ skipping 70 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',
         ],
     }
 }