Move NSScrollerImpDetails to platform/mac/

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.cpp',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
       'Clock.h',
       'ColorChooser.cpp',
@@ skipping 348 matching lines @@
       'graphics/SkSizeHash.h',
       'graphics/TextRun.cpp',
       'graphics/TextRun.h',
       'graphics/TextRunIterator.h',
       'graphics/ThreadSafeDataTransport.cpp',
       'graphics/ThreadSafeDataTransport.h',
       'graphics/WindRule.h',
       'mac/KillRingMac.mm',
       'mac/BlockExceptions.h',
       'mac/BlockExceptions.mm',
+      'mac/NSScrollerImpDetails.h',
+      'mac/NSScrollerImpDetails.mm',
       'mac/WebFontCache.h',
       'mac/WebFontCache.mm',
       'mediastream/MediaConstraints.cpp',
       'mediastream/MediaConstraints.h',
       'mediastream/MediaStreamSource.cpp',
       'mediastream/MediaStreamSource.h',
       'mediastream/RTCConfiguration.h',
       'mediastream/RTCDTMFSenderHandler.cpp',
       'mediastream/RTCDTMFSenderHandler.h',
       'mediastream/RTCDTMFSenderHandlerClient.h',
@@ skipping 122 matching lines @@
       'geometry/RoundedRectTest.cpp',
       'graphics/ThreadSafeDataTransportTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/BidiResolverTest.cpp',
       'text/DateTimeFormatTest.cpp',
     ],
   },
 }