Fix EditingStyle::mergeStyle()'s handling of custom properties

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 4021 matching lines @@
             '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/custom/CustomElementsRegistryTest.cpp',
             'dom/shadow/FlatTreeTraversalTest.cpp',
             'editing/EditingCommandTest.cpp',
             'editing/EditingStrategyTest.cpp',
+            'editing/EditingStyleTest.cpp',
             'editing/EditingTestBase.cpp',
             'editing/EditingTestBase.h',
             'editing/EditingUtilitiesTest.cpp',
             'editing/EditorTest.cpp',
             'editing/FrameSelectionTest.cpp',
             'editing/GranularityStrategyTest.cpp',
             'editing/InputMethodControllerTest.cpp',
             'editing/iterators/BackwardsTextBufferTest.cpp',
             'editing/iterators/CharacterIteratorTest.cpp',
             'editing/iterators/ForwardsTextBufferTest.cpp',
@@ skipping 141 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }