avoid GCC content::CacheStorage::kSizeUnknown redeclaration error

content/browser/cache_storage/cache_storage.h

 // 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.
 
 #ifndef CONTENT_BROWSER_CACHE_STORAGE_CACHE_STORAGE_H_
 #define CONTENT_BROWSER_CACHE_STORAGE_CACHE_STORAGE_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 21 matching lines @@
 class BlobStorageContext;
 }
 
 namespace content {
 class CacheStorageCacheHandle;
 class CacheStorageIndex;
 class CacheStorageScheduler;
 
 // TODO(jkarlin): Constrain the total bytes used per origin.
 
+const int64_t kCacheStorageSizeUnknown = -1;
+
 // CacheStorage holds the set of caches for a given origin. It is
 // owned by the CacheStorageManager. This class expects to be run
 // on the IO thread. The asynchronous methods are executed serially.
 class CONTENT_EXPORT CacheStorage : public CacheStorageCacheObserver {
  public:
-  constexpr static int64_t kSizeUnknown = -1;
-
   typedef base::Callback<void(bool, CacheStorageError)> BoolAndErrorCallback;
   typedef base::Callback<void(std::unique_ptr<CacheStorageCacheHandle>,
                               CacheStorageError)>
       CacheAndErrorCallback;
   using IndexCallback = base::Callback<void(const CacheStorageIndex&)>;
   using SizeCallback = base::Callback<void(int64_t)>;
 
   static const char kIndexFileName[];
 
   CacheStorage(
@@ skipping 206 matching lines @@
   base::CancelableClosure index_write_task_;
 
   base::WeakPtrFactory<CacheStorage> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(CacheStorage);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_CACHE_STORAGE_CACHE_STORAGE_H_