avoid GCC content::CacheStorage::kSizeUnknown redeclaration error

content/browser/cache_storage/cache_storage.h

Index: content/browser/cache_storage/cache_storage.h
diff --git a/content/browser/cache_storage/cache_storage.h b/content/browser/cache_storage/cache_storage.h
index b99886a78a442951602269aab1fffcb547a1e61b..017c425ce4adf56b272dbc02391e668d9cd58e7b 100644
--- a/content/browser/cache_storage/cache_storage.h
+++ b/content/browser/cache_storage/cache_storage.h
@@ -39,13 +39,13 @@ 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)>