Fix FD leak in POSIX Env.

Deleting a PosixWritableFile without calling Close() leaks the file
descriptor. While the API description in include/leveldb/env.h does not
specify whether the caller is responsible for Close()ing the file before
deleting it, all other Env file implementations do release underlying
resources when destroyed, even if Close() is not called.

The leak shows up when running db_tests on Mac Travis, or on a vanilla
MacOS install.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170906843

util/env_posix.cc

@@ -230,7 +230,7 @@ class PosixWritableFile : public WritableFile {
   ~PosixWritableFile() {
     if (fd_ >= 0) {
       // Ignoring any potential errors
-      FlushBuffered();
+      Close();
     }
   }