Move PlatformScreen, PlatformScreenChromium, and Widget 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 30 matching lines @@
       'PODIntervalTree.h',
       'PODRedBlackTree.h',
       'Partitions.cpp',
       'Partitions.h',
       'PasteMode.h',
       'PlatformEvent.h',
       'PlatformGestureEvent.h',
       'PlatformKeyboardEvent.cpp',
       'PlatformKeyboardEvent.h',
       'PlatformMouseEvent.h',
+      'PlatformScreen.cpp',
+      'PlatformScreen.h',
       'PlatformThreadData.cpp',
       'PlatformThreadData.h',
       'PlatformTouchEvent.h',
       'PlatformTouchPoint.h',
       'PlatformWheelEvent.h',
       'PurgeableBuffer.cpp',
       'PurgeableBuffer.h',
       'SSLKeyGenerator.cpp',
       'SSLKeyGenerator.h',
       'SecureTextInput.cpp',
@@ skipping 255 matching lines @@
       'clipboard/ClipboardUtilitiesTest.cpp',
       'geometry/RegionTest.cpp',
       'geometry/RoundedRectTest.cpp',
       'testing/ArenaTestHelpers.h',
       'testing/TreeTestHelpers.cpp',
       'testing/TreeTestHelpers.h',
       'text/DateTimeFormatTest.cpp',
     ],
   },
 }