Restore old behavior where "Sans" was used as fallback font

Source/platform/fonts/AlternateFontFamily.h

 /*
  * Copyright (C) 2006, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2013 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
@@ skipping 99 matching lines @@
     DEFINE_STATIC_LOCAL(const AtomicString, sansStr, ("Sans", AtomicString::ConstructFromLiteral));
     DEFINE_STATIC_LOCAL(const AtomicString, serifStr, ("Serif", AtomicString::ConstructFromLiteral));
     DEFINE_STATIC_LOCAL(const AtomicString, monospaceStr, ("Monospace", AtomicString::ConstructFromLiteral));
 
     switch (description.genericFamily()) {
     case FontDescription::SerifFamily:
         return serifStr;
     case FontDescription::MonospaceFamily:
         return monospaceStr;
     case FontDescription::SansSerifFamily:
+    default:
         return sansStr;
-    default:
-        return AtomicString();
     }
 }
 
 } // namespace WebCore
 
 #endif // AlternateFontFamily_h