avoid GCC content::CacheStorage::kSizeUnknown redeclaration error

content/browser/cache_storage/cache_storage.cc

Index: content/browser/cache_storage/cache_storage.cc
diff --git a/content/browser/cache_storage/cache_storage.cc b/content/browser/cache_storage/cache_storage.cc
index d32fc3d96b9af3e54d9e5920c0f75415ff27e574..b565cc0a8135bc0d8ab32d527a8ba57b8388486a 100644
--- a/content/browser/cache_storage/cache_storage.cc
+++ b/content/browser/cache_storage/cache_storage.cc
@@ -59,7 +59,7 @@ 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;