Move PlatformScreen, PlatformScreenChromium, and Widget 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 2674 matching lines @@
             'platform/chromium/ChromiumDataObject.cpp',
             'platform/chromium/ChromiumDataObject.h',
             'platform/chromium/ChromiumDataObjectItem.cpp',
             'platform/chromium/ChromiumDataObjectItem.h',
             'platform/chromium/FramelessScrollView.cpp',
             'platform/chromium/FramelessScrollView.h',
             'platform/chromium/FramelessScrollViewClient.h',
             'platform/chromium/HistogramSupportChromium.cpp',
             'platform/chromium/MemoryUsageSupportChromium.cpp',
             'platform/chromium/MIMETypeRegistryChromium.cpp',
-            'platform/chromium/PlatformScreenChromium.cpp',
             'platform/chromium/PlatformThemeChromiumDefault.cpp',
             'platform/chromium/PlatformThemeChromiumDefault.h',
             'platform/graphics/BitmapImage.cpp',
             'platform/graphics/ContentDecryptionModule.cpp',
             'platform/graphics/ContentDecryptionModule.h',
             'platform/graphics/ContentDecryptionModuleSession.cpp',
             'platform/graphics/ContentDecryptionModuleSession.h',
             'platform/graphics/Color.cpp',
             'platform/graphics/CrossfadeGeneratedImage.cpp',
             'platform/graphics/CrossfadeGeneratedImage.h',
@@ skipping 952 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++"'
               }],
          ],
     }
 }