Move RefCountedSupplement to platform/

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.cpp',
       'CalculationValue.h',
       'Clock.cpp',
       'Clock.h',
       'ContentType.cpp',
       'ContentType.h',
@@ skipping 59 matching lines @@
       'PlatformThreadData.cpp',
       'PlatformThreadData.h',
       'PlatformTouchEvent.h',
       'PlatformTouchPoint.h',
       'PlatformWheelEvent.h',
       'Prerender.cpp',
       'Prerender.h',
       'PrerenderClient.h',
       'PurgeableBuffer.cpp',
       'PurgeableBuffer.h',
+      'RefCountedSupplement.h',
       'SSLKeyGenerator.cpp',
       'SSLKeyGenerator.h',
       'SecureTextInput.cpp',
       'SecureTextInput.h',
       'SerializedResource.h',
       'SharedBuffer.cpp',
       'SharedBuffer.h',
       'SharedBufferChunkReader.cpp',
       'SharedBufferChunkReader.h',
       'SharedTimer.cpp',
@@ skipping 322 matching lines @@
       'geometry/RoundedRectTest.cpp',
       'graphics/ThreadSafeDataTransportTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/BidiResolverTest.cpp',
       'text/DateTimeFormatTest.cpp',
     ],
   },
 }