Move CrossThreadCopier.cpp/.h to platform/

Source/core/core.gypi

 {
     'variables': {
         'core_idl_files': [
             'css/CSS.idl',
             'css/CSSCharsetRule.idl',
             'css/CSSFontFaceLoadEvent.idl',
             'css/CSSFontFaceRule.idl',
             'css/CSSHostRule.idl',
             'css/CSSImportRule.idl',
             'css/CSSKeyframeRule.idl',
@@ skipping 2604 matching lines @@
             'html/track/WebVTTParser.cpp',
             'html/track/WebVTTParser.h',
             'html/track/WebVTTToken.h',
             'html/track/WebVTTTokenizer.cpp',
             'html/track/WebVTTTokenizer.h',
         ],
         'webcore_platform_files': [
             'platform/AsyncFileSystemCallbacks.h',
             'platform/ContextMenu.cpp',
             'platform/ContextMenuItem.cpp',
-            'platform/CrossThreadCopier.cpp',
-            'platform/CrossThreadCopier.h',
             'platform/Cursor.cpp',
             'platform/DragData.cpp',
             'platform/DragImage.cpp',
             'platform/HashTools.h',
             'platform/KillRing.h',
             'platform/KillRingNone.cpp',
             'platform/LifecycleContext.cpp',
             'platform/LifecycleContext.h',
             'platform/LifecycleNotifier.cpp',
             'platform/LifecycleNotifier.h',
@@ skipping 1012 matching lines @@
                 'bison_exe': 'bison',
                 # We specify a preprocess so it happens locally and won't get
                 # distributed to goma.
                 # FIXME: /usr/bin/gcc won't exist on OSX forever. We want to
                 # use /usr/bin/clang once we require Xcode 4.x.
                 'preprocessor': '--preprocessor "/usr/bin/gcc -E -P -x c++"'
               }],
          ],
     }
 }