avoid GCC content::CacheStorage::kSizeUnknown redeclaration error

content/browser/cache_storage/cache_storage.cc

 // Copyright 2014 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 "content/browser/cache_storage/cache_storage.h"
 
 #include <stddef.h>
 
 #include <set>
 #include <string>
@@ skipping 41 matching lines @@
                                 const CacheStorage::SizeCallback& callback) {
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(callback, *accumulator));
 }
 
 void DoNothingWithBool(bool success) {}
 
 }  // namespace
 
 const char CacheStorage::kIndexFileName[] = "index.txt";
-const int64_t CacheStorage::kSizeUnknown;
+constexpr int64_t CacheStorage::kSizeUnknown;
 
 struct CacheStorage::CacheMatchResponse {
   CacheMatchResponse() = default;
   ~CacheMatchResponse() = default;
 
   CacheStorageError error;
   std::unique_ptr<ServiceWorkerResponse> service_worker_response;
   std::unique_ptr<storage::BlobDataHandle> blob_data_handle;
 };
 
@@ skipping 1023 matching lines @@
         GetLoadedCache(cache_metadata.name);
     CacheStorageCache* cache = cache_handle->value();
     cache->Size(base::Bind(&CacheStorage::SizeRetrievedFromCache,
                            weak_factory_.GetWeakPtr(),
                            base::Passed(std::move(cache_handle)),
                            barrier_closure, accumulator_ptr));
   }
 }
 
 }  // namespace content