IndexedDB: Simplify WebIDBCallbacks exports, strip IDBFactoryBackend

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'ApplicationCacheHost.cpp',
       'ApplicationCacheHostInternal.h',
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'AutofillPopupMenuClient.cpp',
       'AutofillPopupMenuClient.h',
@@ skipping 141 matching lines @@
       'WebGeolocationController.cpp',
       'WebGeolocationError.cpp',
       'WebGeolocationPermissionRequest.cpp',
       'WebGeolocationPermissionRequestManager.cpp',
       'WebGeolocationPosition.cpp',
       'WebGlyphCache.cpp',
       'WebHelperPluginImpl.cpp',
       'WebHelperPluginImpl.h',
       'WebHistoryItem.cpp',
       'WebHitTestResult.cpp',
-      'WebIDBCallbacks.cpp',
       'WebIDBDatabaseError.cpp',
       'WebIDBKey.cpp',
       'WebIDBKeyPath.cpp',
       'WebIDBKeyRange.cpp',
       'WebIDBMetadata.cpp',
       'WebImageCache.cpp',
       'WebImageDecoder.cpp',
       'WebImageSkia.cpp',
       'WebInputElement.cpp',
       'WebInputEvent.cpp',
@@ skipping 182 matching lines @@
         {
           'web_unittest_files': [
             'tests/KeyCodeConversionTestGtk.cpp',
             'tests/WebInputEventFactoryTestGtk.cpp',
           ],
         }
       ],
     ],
   },
 }