experiment with -Wimplicit-fallthrough

net/http/http_cache_transaction.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/http/http_cache_transaction.h"
 
 #include "build/build_config.h"
 
 #if defined(OS_POSIX)
 #include <unistd.h>
@@ skipping 404 matching lines @@
   int rv;
 
   switch (mode_) {
     case READ_WRITE:
       DCHECK(partial_.get());
       if (!network_trans_.get()) {
         // We are just reading from the cache, but we may be writing later.
         rv = ReadFromEntry(buf, buf_len);
         break;
       }
+      FALLTHROUGH_INTENDED;
     case NONE:
     case WRITE:
       DCHECK(network_trans_.get());
       rv = ReadFromNetwork(buf, buf_len);
       break;
     case READ:
       rv = ReadFromEntry(buf, buf_len);
       break;
     default:
       NOTREACHED();
@@ skipping 2100 matching lines @@
       UMA_HISTOGRAM_PERCENTAGE("HttpCache.PercentBeforeSend.Updated",
                                before_send_percent);
       break;
     }
     default:
       NOTREACHED();
   }
 }
 
 }  // namespace net