Remove legacy WebPageOverlay method.

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'ChromeClientImpl.cpp',
       'ChromeClientImpl.h',
       'ColorChooserPopupUIController.cpp',
       'ColorChooserPopupUIController.h',
@@ skipping 181 matching lines @@
       'WebMIDIClientMock.cpp',
       'WebMIDIPermissionRequest.cpp',
       'WebMediaDevicesRequest.cpp',
       'WebMediaPlayerClientImpl.cpp',
       'WebMediaPlayerClientImpl.h',
       'WebMediaStreamRegistry.cpp',
       'WebNetworkStateNotifier.cpp',
       'WebNode.cpp',
       'WebNodeList.cpp',
       'WebOptionElement.cpp',
-      'WebPageOverlay.cpp',
       'WebPagePopupImpl.cpp',
       'WebPagePopupImpl.h',
       'WebPageSerializer.cpp',
       'WebPageSerializerImpl.cpp',
       'WebPageSerializerImpl.h',
       'WebPerformance.cpp',
       'WebPluginContainerImpl.cpp',
       'WebPluginContainerImpl.h',
       'WebPluginDocument.cpp',
       'WebPluginLoadObserver.cpp',
@@ skipping 142 matching lines @@
       ['os_posix==1 and OS!="mac"',
         {
           'web_unittest_files': [
             'tests/LocaleICUTest.cpp',
           ],
         }
       ],
     ],
   },
 }