Fix compile error caused in LDB tool

Summary:
Fixed the following compile error.
    tools/reduce_levels_test.cc:89:31: error: no matching function for call to 'InitFromCmdLineArgs'
      LDBCommand* level_reducer = LDBCommand::InitFromCmdLineArgs(args);
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ./util/ldb_cmd.h:56:22: note: candidate function not viable: requires 3 arguments, but 1 was provided
      static LDBCommand* InitFromCmdLineArgs(
                         ^
    ./util/ldb_cmd.h:62:22: note: candidate function not viable: requires 4 arguments, but 1 was provided
      static LDBCommand* InitFromCmdLineArgs(
                         ^
    1 error generated.

Test Plan:
make reduce_levels_test
./reduce_levels_test

Reviewers: haobo, ljin, sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19251

util/ldb_cmd.h

@@ -55,15 +55,15 @@ public:
 
   static LDBCommand* InitFromCmdLineArgs(
     const vector<string>& args,
-    const Options& options,
+    const Options& options = Options(),
-    const LDBOptions& ldb_options
+    const LDBOptions& ldb_options = LDBOptions()
   );
 
   static LDBCommand* InitFromCmdLineArgs(
     int argc,
     char** argv,
-    const Options& options,
+    const Options& options = Options(),
-    const LDBOptions& ldb_options
+    const LDBOptions& ldb_options = LDBOptions()
   );
 
   bool ValidateCmdLineOptions();