Remove InitOnce from the port API.

This is not an API-breaking change, because it reduces the API that the
leveldb embedder must implement. The project will build just fine
against ports that still implement InitOnce.

C++11 guarantees thread-safe initialization of static variables inside
functions. This is a more restricted form of std::call_once or
pthread_once_t (e.g., single call site), so the compiler might be able
to generate better code [1]. Equally important, having less code in
port_example.h makes it easier to port to other platforms.

Due to the change above, this CL introduces a new approach for storing
the singleton BytewiseComparatorImpl instance returned by
BytewiseComparator(). The new approach avoids a dynamic memory
allocation, which eliminates the false positive from LeakSanitizer
reported in https://github.com/google/leveldb/issues/200

[1] https://stackoverflow.com/a/27206650/

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212348004

util/no_destructor_test.cc

@@ -0,0 +1,49 @@
+// Copyright (c) 2018 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#include <cstdint>
+#include <cstdlib>
+#include <utility>
+
+#include "util/no_destructor.h"
+#include "util/testharness.h"
+
+namespace leveldb {
+
+namespace {
+
+struct DoNotDestruct {
+ public:
+  DoNotDestruct(uint32_t a, uint64_t b) : a(a), b(b) {}
+  ~DoNotDestruct() { std::abort(); }
+
+  // Used to check constructor argument forwarding.
+  uint32_t a;
+  uint64_t b;
+};
+
+constexpr const uint32_t kGoldenA = 0xdeadbeef;
+constexpr const uint64_t kGoldenB = 0xaabbccddeeffaabb;
+
+}  // namespace
+
+class NoDestructorTest { };
+
+TEST(NoDestructorTest, StackInstance) {
+  NoDestructor<DoNotDestruct> instance(kGoldenA, kGoldenB);
+  ASSERT_EQ(kGoldenA, instance.get()->a);
+  ASSERT_EQ(kGoldenB, instance.get()->b);
+}
+
+TEST(NoDestructorTest, StaticInstance) {
+  static NoDestructor<DoNotDestruct> instance(kGoldenA, kGoldenB);
+  ASSERT_EQ(kGoldenA, instance.get()->a);
+  ASSERT_EQ(kGoldenB, instance.get()->b);
+}
+
+}  // namespace leveldb
+
+int main(int argc, char** argv) {
+  return leveldb::test::RunAllTests();
+}