Move ScrollbarThemeClient to Source/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 2629 matching lines @@
             'platform/ScrollAnimatorNone.h',
             'platform/ScrollView.cpp',
             'platform/ScrollableArea.cpp',
             'platform/Scrollbar.cpp',
             'platform/Scrollbar.h',
             'platform/ScrollbarTheme.cpp',
             'platform/ScrollbarTheme.h',
             'platform/ScrollbarThemeAndroid.cpp',
             'platform/ScrollbarThemeAuraOrGtk.cpp',
             'platform/ScrollbarThemeAuraOrGtk.h',
-            'platform/ScrollbarThemeClient.h',
             'platform/ScrollbarThemeMacCommon.h',
             'platform/ScrollbarThemeMacCommon.mm',
             'platform/ScrollbarThemeMacNonOverlayAPI.h',
             'platform/ScrollbarThemeMacNonOverlayAPI.mm',
             'platform/ScrollbarThemeMacOverlayAPI.h',
             'platform/ScrollbarThemeMacOverlayAPI.mm',
             'platform/ScrollbarThemeNonMacCommon.cpp',
             'platform/ScrollbarThemeNonMacCommon.h',
             'platform/ScrollbarThemeOverlay.cpp',
             'platform/ScrollbarThemeOverlay.h',
@@ skipping 992 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++"'
               }],
          ],
     }
 }