Clarify that BlockSizeForFileType can't be called with EXTERNAL FileType

net/disk_cache/blockfile/addr.h

Index: net/disk_cache/blockfile/addr.h
diff --git a/net/disk_cache/blockfile/addr.h b/net/disk_cache/blockfile/addr.h
index f21fbf2aa14000ccdd6f978a976ed238b4bc3099..32e035eac99aa66432653bc99a238016345e325e 100644
--- a/net/disk_cache/blockfile/addr.h
+++ b/net/disk_cache/blockfile/addr.h
@@ -11,6 +11,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "base/logging.h"
 #include "net/base/net_export.h"
 #include "net/disk_cache/blockfile/disk_format_base.h"
 
@@ -138,9 +139,11 @@ class NET_EXPORT_PRIVATE Addr {
         return 104;
       case BLOCK_EVICTED:
         return 48;
-      default:
+      case EXTERNAL:
+        NOTREACHED();
         return 0;
     }
+    return 0;
   }
 
   static FileType RequiredFileType(int size) {