Backport https://codereview.chromium.org/64243002/ to fix a renderer crash.
--- third_party/WebKit/Source/platform/fonts/AlternateFontFamily.h.orig	2013-11-07 18:46:58.174333486 +0000
+++ third_party/WebKit/Source/platform/fonts/AlternateFontFamily.h	2013-11-07 18:47:13.974434037 +0000
@@ -117,9 +117,8 @@
     case FontDescription::MonospaceFamily:
         return monospaceStr;
     case FontDescription::SansSerifFamily:
-        return sansStr;
     default:
-        return AtomicString();
+        return sansStr;
     }
 }