Move DragImage from core/platform/ to platform/

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.cpp',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
       'Clock.h',
       'ColorChooser.cpp',
@@ skipping 12 matching lines @@
       'Cursor.cpp',
       'Cursor.h',
       'DateComponents.cpp',
       'DateComponents.h',
       'DateTimeChooser.cpp',
       'DateTimeChooser.h',
       'DateTimeChooserClient.cpp',
       'DateTimeChooserClient.h',
       'Decimal.cpp',
       'Decimal.h',
+      'DragImage.cpp',
+      'DragImage.h',
       'EventTracer.cpp',
       'EventTracer.h',
       'FileChooser.cpp',
       'FileChooser.h',
       'FileMetadata.cpp',
       'FileMetadata.h',
       'FileSystemType.h',
       'FloatConversion.h',
       'HostWindow.h',
       'JSONValues.cpp',
@@ skipping 861 matching lines @@
       'text/BidiResolverTest.cpp',
       'text/DateTimeFormatTest.cpp',
       'text/UnicodeUtilitiesTest.cpp',
       'weborigin/DatabaseIdentifierTest.cpp',
       'weborigin/KURLTest.cpp',
       'weborigin/OriginAccessEntryTest.cpp',
       'weborigin/SecurityOriginTest.cpp',
     ],
   },
 }