Minor fix to GFLAGS usage in persistent cache

Summary:
The general convention in RocksDB is to use GFLAGS instead of google. Fixing the anomaly.
Closes https://github.com/facebook/rocksdb/pull/1470

Differential Revision: D4149213

Pulled By: kradhakrishnan

fbshipit-source-id: 2dafa53

utilities/persistent_cache/hash_table_bench.cc

@@ -272,9 +272,9 @@ class GranularLockImpl : public HashTableImpl<size_t, string> {
 // main
 //
 int main(int argc, char** argv) {
-  google::SetUsageMessage(std::string("\nUSAGE:\n") + std::string(argv[0]) +
+  GFLAGS::SetUsageMessage(std::string("\nUSAGE:\n") + std::string(argv[0]) +
                           " [OPTIONS]...");
-  google::ParseCommandLineFlags(&argc, &argv, false);
+  GFLAGS::ParseCommandLineFlags(&argc, &argv, false);
 
   //
   // Micro benchmark unordered_map

utilities/persistent_cache/persistent_cache_bench.cc

@@ -302,9 +302,9 @@ class CacheTierBenchmark {
 // main
 //
 int main(int argc, char** argv) {
-  google::SetUsageMessage(std::string("\nUSAGE:\n") + std::string(argv[0]) +
+  GFLAGS::SetUsageMessage(std::string("\nUSAGE:\n") + std::string(argv[0]) +
                           " [OPTIONS]...");
-  google::ParseCommandLineFlags(&argc, &argv, false);
+  GFLAGS::ParseCommandLineFlags(&argc, &argv, false);
 
   std::ostringstream msg;
   msg << "Config" << std::endl