Move ScrollbarThemeClient to Source/platform

Source/platform/blink_platform.gypi

 {
   'variables': {
     'platform_files': [
       'Clock.cpp',
       'Clock.h',
       'ContentType.cpp',
       'ContentType.h',
       'DateComponents.cpp',
       'DateComponents.h',
       'Decimal.cpp',
@@ skipping 192 matching lines @@
       'network/ProxyServer.cpp',
       'network/ProxyServer.h',
       'network/ResourceError.cpp',
       'network/ResourceError.h',
       'network/SocketStreamError.cpp',
       'network/SocketStreamError.h',
       'network/SocketStreamHandle.cpp',
       'network/SocketStreamHandle.h',
       'network/SocketStreamHandleClient.h',
       'network/SocketStreamHandleInternal.h',
+      'scroll/ScrollbarThemeClient.h',
       'scroll/ScrollTypes.h',
       'text/BidiContext.cpp',
       'text/BidiContext.h',
       'text/BidiResolver.h',
       'text/BidiRunList.h',
       'text/DateTimeFormat.cpp',
       'text/DateTimeFormat.h',
       'text/DecodeEscapeSequences.h',
       'text/LineEnding.cpp',
       'text/LineEnding.h',
@@ skipping 54 matching lines @@
       'geometry/RoundedRectTest.cpp',
       'geometry/RoundedRectTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/DateTimeFormatTest.cpp',
       'text/DateTimeFormatTest.cpp',
     ],
   },
 }