Consolidate ReadFileToString() (#6366)

Summary:
It's a minor refactoring. We have two ReadFileToString() but they are very similar. Make the one with Env argument calls the one with FS argument instead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6366

Test Plan: Run all existing tests

Differential Revision: D19712332

fbshipit-source-id: 5ae6fabf6355938690d95cda52afd1f39e0a7823

env/env.cc

@@ -378,28 +378,8 @@ Status WriteStringToFile(Env* env, const Slice& data, const std::string& fname,
 }
 
 Status ReadFileToString(Env* env, const std::string& fname, std::string* data) {
-  EnvOptions soptions;
-  data->clear();
-  std::unique_ptr<SequentialFile> file;
-  Status s = env->NewSequentialFile(fname, &file, soptions);
-  if (!s.ok()) {
-    return s;
-  }
-  static const int kBufferSize = 8192;
-  char* space = new char[kBufferSize];
-  while (true) {
-    Slice fragment;
-    s = file->Read(kBufferSize, &fragment, space);
-    if (!s.ok()) {
-      break;
-    }
-    data->append(fragment.data(), fragment.size());
-    if (fragment.empty()) {
-      break;
-    }
-  }
-  delete[] space;
-  return s;
+  LegacyFileSystemWrapper lfsw(env);
+  return ReadFileToString(&lfsw, fname, data);
 }
 
 EnvWrapper::~EnvWrapper() {